diff --git a/Array/ContainsDuplicateII.swift b/Array/ContainsDuplicateII.swift
index 8a820f08..1bca20f6 100644
--- a/Array/ContainsDuplicateII.swift
+++ b/Array/ContainsDuplicateII.swift
@@ -16,7 +16,7 @@ class ContainsDuplicateII {
         // key: nums[index], value: index
         var dict = [Int: Int]()
         
-        for i in 0 ..< nums.count {
+        for i in 0..<nums.count {
             guard let index = dict[nums[i]] where i - index <= k else {
                 dict[nums[i]] = i
                 continue
diff --git a/Array/FourSum.swift b/Array/FourSum.swift
index 4a46f09b..081d3642 100644
--- a/Array/FourSum.swift
+++ b/Array/FourSum.swift
@@ -6,8 +6,8 @@
  */
 
 class FourSum {
-    func fourSum(nums: [Int], _ target: Int) -> [[Int]] {
-        let nums = nums.sort({$0 < $1})
+    func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
+        let nums = nums.sorted(by: <)
         var threeSum = 0
         var twoSum = 0
         var left = 0
@@ -18,13 +18,13 @@ class FourSum {
             return res
         }
         
-        for i in 0 ..< nums.count - 3 {
+        for i in 0..<nums.count - 3 {
             guard i == 0 || nums[i] != nums[i - 1] else {
                 continue
             }
             threeSum = target - nums[i]
             
-            for j in i + 1 ..< nums.count - 2 {
+            for j in i + 1..<nums.count - 2 {
                 guard j == i + 1 || nums[j] != nums[j - 1] else {
                     continue
                 }
diff --git a/Array/GameLife.swift b/Array/GameLife.swift
index e2827661..510cbef1 100644
--- a/Array/GameLife.swift
+++ b/Array/GameLife.swift
@@ -15,8 +15,8 @@ class GameLife {
     
         let m = board.count, n = board[0].count
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 changeStatus(&board, i, j, m, n)
             }
         }
@@ -27,8 +27,8 @@ class GameLife {
     private func changeStatus(_ board: inout [[Int]], _ i: Int, _ j: Int, _ m: Int, _ n: Int) {
         var liveNum = 0
     
-        for x in i - 1 ... i + 1 {
-            for y in j - 1 ... j + 1 {
+        for x in i - 1...i + 1 {
+            for y in j - 1...j + 1 {
                 if x < 0 || x >= m || y < 0 || y >= n {
                     continue
                 }
diff --git a/Array/IntersectionTwoArraysII.swift b/Array/IntersectionTwoArraysII.swift
index 9aa109a1..42bf664d 100644
--- a/Array/IntersectionTwoArraysII.swift
+++ b/Array/IntersectionTwoArraysII.swift
@@ -9,8 +9,8 @@
 
  class IntersectionTwoArraysII {
     func intersect(nums1: [Int], _ nums2: [Int]) -> [Int] {
-        var nums1 = nums1.sort({$0 < $1})
-        var nums2 = nums2.sort({$0 < $1})
+        var nums1 = nums1.sorted(by: <)
+        var nums2 = nums2.sorted(by: <)
         
         var i = 0
         var j = 0
diff --git a/Array/ProductExceptSelf.swift b/Array/ProductExceptSelf.swift
index 3cd12312..b87a3581 100644
--- a/Array/ProductExceptSelf.swift
+++ b/Array/ProductExceptSelf.swift
@@ -16,7 +16,7 @@ class ProductExceptSelf {
         let left = _initLeft(nums)
         let right = _initRight(nums)
         
-        for i in 0 ..< nums.count {
+        for i in 0..<nums.count {
             res.append(left[i] * right[i])
         }
         
@@ -27,7 +27,7 @@ class ProductExceptSelf {
         var left = [Int]()
         left.append(1)
         
-        for i in 1 ..< nums.count {
+        for i in 1..<nums.count {
             left.append(left[i - 1] * nums[i - 1])
         }
         
diff --git a/Array/RemoveDuplicatesFromSortedArrayII.swift b/Array/RemoveDuplicatesFromSortedArrayII.swift
index dbbe392b..4575db94 100644
--- a/Array/RemoveDuplicatesFromSortedArrayII.swift
+++ b/Array/RemoveDuplicatesFromSortedArrayII.swift
@@ -14,7 +14,7 @@ class RemoveDuplicatesFromSortedArrayII {
         }
         
         var lastIndex = 1
-        for i in 2 ..< nums.count {
+        for i in 2..<nums.count {
             if nums[lastIndex] != nums[i] || nums[lastIndex] != nums[lastIndex - 1] {
                 lastIndex += 1
                 nums[lastIndex] = nums[i]
diff --git a/Array/RotateImage.swift b/Array/RotateImage.swift
index a631e6d7..fb2538d5 100644
--- a/Array/RotateImage.swift
+++ b/Array/RotateImage.swift
@@ -9,9 +9,9 @@ class RotateImage {
     func rotate(_ matrix: inout [[Int]]) {
         let n = matrix.count
         
-        for layer in 0 ..< n / 2 {
+        for layer in 0..<n / 2 {
             let start = layer, end = n - layer - 1
-            for i in start ..< end {
+            for i in start..<end {
                 let offset = i - start
                 
                 (matrix[start][i], matrix[i][end], matrix[end][end - offset], matrix[end - offset][start]) = (matrix[end - offset][start], matrix[start][i], matrix[i][end], matrix[end][end - offset])
diff --git a/Array/SetMatrixZeroes.swift b/Array/SetMatrixZeroes.swift
index 8c6bef5e..a8690139 100644
--- a/Array/SetMatrixZeroes.swift
+++ b/Array/SetMatrixZeroes.swift
@@ -11,18 +11,18 @@ class SetMatrixZeroes {
         var rowHasZero = false, colHasZero = false
         let m = matrix.count, n = matrix[0].count
   
-        for i in 0 ..< m where matrix[i][0] == 0 {
+        for i in 0..<m where matrix[i][0] == 0 {
             colHasZero = true
             break
         }
   
-        for i in 0 ..< n where matrix[0][i] == 0 {
+        for i in 0..<n where matrix[0][i] == 0 {
             rowHasZero = true
             break
         }
   
-        for i in 1 ..< m {
-            for j in 1 ..< n {
+        for i in 1..<m {
+            for j in 1..<n {
                 if matrix[i][j] == 0 {
                     matrix[0][j] = 0
                     matrix[i][0] = 0
@@ -30,8 +30,8 @@ class SetMatrixZeroes {
             }
         }
   
-        for i in 1 ..< m {
-            for j in 1 ..< n {
+        for i in 1..<m {
+            for j in 1..<n {
                 if matrix[0][j] == 0 || matrix[i][0] == 0 {
                     matrix[i][j] = 0
                 }
@@ -39,13 +39,13 @@ class SetMatrixZeroes {
         }
   
         if rowHasZero {
-            for i in 0 ..< n {
+            for i in 0..<n {
                 matrix[0][i] = 0
             }
         }
   
         if colHasZero {
-            for i in 0 ..< m {
+            for i in 0..<m {
                 matrix[i][0] = 0
             }
         }
diff --git a/Array/SlidingWindowMaximum.swift b/Array/SlidingWindowMaximum.swift
index ca6c73db..4cd94c9a 100644
--- a/Array/SlidingWindowMaximum.swift
+++ b/Array/SlidingWindowMaximum.swift
@@ -12,7 +12,7 @@ class SlidingWindowMaximum {
         var maxIdx = [Int]()
         var res = [Int]()
         
-        for i in 0 ..< nums.count {
+        for i in 0..<nums.count {
             while maxIdx.count > 0 && nums[maxIdx.last!] < nums[i] {
                 maxIdx.removeLast()
             }
diff --git a/Array/SpiralMatrix.swift b/Array/SpiralMatrix.swift
index 18dda780..b1522a80 100644
--- a/Array/SpiralMatrix.swift
+++ b/Array/SpiralMatrix.swift
@@ -20,7 +20,7 @@ class SpiralMatrix {
         
         while true {
             // top
-            for i in startY ... endY {
+            for i in startY...endY {
                 res.append(matrix[startX][i])
             }
             startX += 1
@@ -29,7 +29,7 @@ class SpiralMatrix {
             }
             
             // right
-            for i in startX ... endX {
+            for i in startX...endX {
                 res.append(matrix[i][endY])
             }
             endY -= 1
diff --git a/Array/SpiralMatrixII.swift b/Array/SpiralMatrixII.swift
index 8619d84b..310aa900 100644
--- a/Array/SpiralMatrixII.swift
+++ b/Array/SpiralMatrixII.swift
@@ -14,18 +14,18 @@ class SpiralMatrixII {
         var num = 1
         var res = Array(repeating: Array(repeating: 0, count: n), count: n)
         
-        for layer in 0 ..< n / 2 {
+        for layer in 0..<n / 2 {
             let start = layer
             let end = n - layer - 1
             
             // top
-            for i in start ..< end {
+            for i in start..<end {
                 res[start][i] = num
                 num += 1
             }
             
             // right
-            for i in start ..< end {
+            for i in start..<end {
                 res[i][end] = num
                 num += 1
             }
diff --git a/Array/SummaryRanges.swift b/Array/SummaryRanges.swift
index 2a401d24..520b4c11 100644
--- a/Array/SummaryRanges.swift
+++ b/Array/SummaryRanges.swift
@@ -16,7 +16,7 @@ class SummaryRanges {
             return res
         }
   
-        for i in 0 ... nums.count {
+        for i in 0...nums.count {
             if i == nums.count || (i > 0 && nums[i] != nums[i - 1] + 1) {
                 str = "\(nums[start])"
                 if i - 1 != start {
diff --git a/Array/ThreeSum.swift b/Array/ThreeSum.swift
index 69f57b5e..8c05de26 100644
--- a/Array/ThreeSum.swift
+++ b/Array/ThreeSum.swift
@@ -7,14 +7,14 @@
 
 class ThreeSum {
     func threeSum(nums: [Int]) -> [[Int]] {
-        var nums = nums.sort({$0 < $1})
+        var nums = nums.sorted(by: <)
         var res = [[Int]]()
         
         if nums.count <= 2 {
             return res
         }
         
-        for i in 0 ... nums.count - 3 {
+        for i in 0...nums.count - 3 {
             if i == 0 || nums[i] != nums[i - 1] {
                 var remain = -nums[i]
                 var left = i + 1
diff --git a/Array/ThreeSumClosest.swift b/Array/ThreeSumClosest.swift
index d2327bcb..78b6db49 100644
--- a/Array/ThreeSumClosest.swift
+++ b/Array/ThreeSumClosest.swift
@@ -12,7 +12,7 @@
   
         let nums = nums.sorted()
   
-        for i in 0 ..< nums.count - 2 {
+        for i in 0..<nums.count - 2 {
             if i == 0 || nums[i] != nums[i - 1] {
                 let twoSum = target - nums[i]
                 var left = i + 1
diff --git a/Array/ValidSudoku.swift b/Array/ValidSudoku.swift
index ce7d79f3..fc738628 100644
--- a/Array/ValidSudoku.swift
+++ b/Array/ValidSudoku.swift
@@ -15,9 +15,9 @@ class ValidSudoku {
     private func _isRowValid(board: [[Character]]) -> Bool {
         var visited = Array(count: size, repeatedValue: false)
         
-        for i in 0 ..< size {
+        for i in 0..<size {
             visited = Array(count: size, repeatedValue: false)
-            for j in 0 ..< size {
+            for j in 0..<size {
                 if !_isValidChar(board[i][j], &visited) {
                     return false
                 }
@@ -30,9 +30,9 @@ class ValidSudoku {
     private func _isColValid(board: [[Character]]) -> Bool {
         var visited = Array(count: size, repeatedValue: false)
 
-        for i in 0 ..< size {
+        for i in 0..<size {
             visited = Array(count: size, repeatedValue: false)
-            for j in 0 ..< size {
+            for j in 0..<size {
                 if !_isValidChar(board[j][i], &visited) {
                     return false
                 }
@@ -48,8 +48,8 @@ class ValidSudoku {
         for i in 0.stride(to: size, by: 3) {
             for j in 0.stride(to: size, by: 3) {
                 visited = Array(count: size, repeatedValue: false)
-                for m in i ..< i + 3 {
-                    for n in j ..< j + 3 {
+                for m in i..<i + 3 {
+                    for n in j..<j + 3 {
                         if !_isValidChar(board[m][n], &visited) {
                             return false
                         }
diff --git a/DFS/CombinationSum.swift b/DFS/CombinationSum.swift
index 69163dec..3ad3aa92 100644
--- a/DFS/CombinationSum.swift
+++ b/DFS/CombinationSum.swift
@@ -11,7 +11,7 @@ class CombinationSum {
         var res = [[Int]]()
         var path = [Int]()
         
-        _dfs(candidates.sort({$0 < $1}), target, &res, &path, 0)
+        _dfs(candidates.sorted(by: <), target, &res, &path, 0)
         
         return res
     }
@@ -22,7 +22,7 @@ class CombinationSum {
             return
         }
         
-        for i in index ..< candidates.count {
+        for i in index..<candidates.count {
             guard candidates[i] <= target else {
                 break
             }
diff --git a/DFS/CombinationSumII.swift b/DFS/CombinationSumII.swift
index 724aed1c..9e25bc3c 100644
--- a/DFS/CombinationSumII.swift
+++ b/DFS/CombinationSumII.swift
@@ -11,7 +11,7 @@ class CombinationSumII {
         var res = [[Int]]()
         var path = [Int]()
         
-        _dfs(&res, &path, target, candidates.sort({$0 < $1}), 0)
+        _dfs(&res, &path, target, candidates.sorted(by: <), 0)
         
         return res
     }
@@ -22,7 +22,7 @@ class CombinationSumII {
             return
         }
         
-        for i in index ..< candidates.count {
+        for i in index..<candidates.count {
             guard candidates[i] <= target else {
                 break
             }
diff --git a/DFS/CombinationSumIII.swift b/DFS/CombinationSumIII.swift
index fb993cfd..6cb7688d 100644
--- a/DFS/CombinationSumIII.swift
+++ b/DFS/CombinationSumIII.swift
@@ -8,7 +8,7 @@
 
 class CombinationSumIII {
     func combinationSum3(k: Int, _ n: Int) -> [[Int]] {
-        let candidates = [Int](1 ... 9)
+        let candidates = [Int](1...9)
         var res = [[Int]]()
         var path = [Int]()
         
@@ -23,7 +23,7 @@ class CombinationSumIII {
             return
         }       
         
-        for i in index ..< candidates.count {
+        for i in index..<candidates.count {
             guard candidates[i] <= target else {
                 break
             }
diff --git a/DFS/Combinations.swift b/DFS/Combinations.swift
index fc935d79..fd0c7403 100644
--- a/DFS/Combinations.swift
+++ b/DFS/Combinations.swift
@@ -10,7 +10,7 @@ class Combinations {
     func combine(n: Int, _ k: Int) -> [[Int]] {
         var res = [[Int]]()
         var path = [Int]()
-        let nums = [Int](1 ... n)
+        let nums = [Int](1...n)
         
         _dfs(nums, &res, &path, 0, k)
         
@@ -23,7 +23,7 @@ class Combinations {
             return
         }
         
-        for i in index ..< nums.count {
+        for i in index..<nums.count {
             path.append(nums[i])
             _dfs(nums, &res, &path, i + 1, k)
             path.removeLast()
diff --git a/DFS/CourseSchedule.swift b/DFS/CourseSchedule.swift
index 3fb907ca..64aa7396 100644
--- a/DFS/CourseSchedule.swift
+++ b/DFS/CourseSchedule.swift
@@ -20,7 +20,7 @@ class CourseSchedule {
         var indegree = [Int](repeatElement(0, count: numCourses))
         
         // 1. Create the graph
-        for i in 0 ..< prerequisites.count {
+        for i in 0..<prerequisites.count {
             let course = prerequisites[i][0]
             let pre = prerequisites[i][1]
             
@@ -34,7 +34,7 @@ class CourseSchedule {
         
         // 3. Create a array of sources
         var sources = [Int]()
-        for i in 0 ..< numCourses {
+        for i in 0..<numCourses {
             if indegree[i] == 0 {
                 sources.append(i)
             }
@@ -49,7 +49,7 @@ class CourseSchedule {
             count += 1
             
             // 4.ii. Decrement the in-degree of the destination node
-            for i in 0 ..< numCourses {
+            for i in 0..<numCourses {
                 if graph[source!][i] == 1 {
                     indegree[i] -= 1
                     // Check all of its destination vertices and add them to the set if they have no incoming edges
diff --git a/DFS/CourseScheduleII.swift b/DFS/CourseScheduleII.swift
index 20cb95dc..9a0f1f14 100644
--- a/DFS/CourseScheduleII.swift
+++ b/DFS/CourseScheduleII.swift
@@ -20,7 +20,7 @@ class CourseScheduleII {
         var indegree = [Int](repeatElement(0, count: numCourses))
         
         // 1. Create the graph
-        for i in 0 ..< prerequisites.count {
+        for i in 0..<prerequisites.count {
             let course = prerequisites[i][0]
             let pre = prerequisites[i][1]
             
@@ -34,7 +34,7 @@ class CourseScheduleII {
         
         // 3. Create a array of sources
         var sources = [Int]()
-        for i in 0 ..< numCourses {
+        for i in 0..<numCourses {
             if indegree[i] == 0 {
                 sources.append(i)
             }
@@ -47,7 +47,7 @@ class CourseScheduleII {
             topoSortedList.append(source!)
             
             // 4.ii. Decrement the in-degree of the destination node
-            for i in 0 ..< numCourses {
+            for i in 0..<numCourses {
                 if graph[source!][i] == 1 {
                     indegree[i] -= 1
                     // Check all of its destination vertices and add them to the set if they have no incoming edges
diff --git a/DFS/ExpressionAddOperators.swift b/DFS/ExpressionAddOperators.swift
index fb183660..b03bd24c 100644
--- a/DFS/ExpressionAddOperators.swift
+++ b/DFS/ExpressionAddOperators.swift
@@ -33,11 +33,11 @@ class ExpressionAddOperators {
             return
         }
         
-        for i in pos ..< numChars.count {
+        for i in pos..<numChars.count {
             if i != pos && numChars[pos] == "0" {
                 break
             }
-            let curt = Int(String(numChars[pos ..< i + 1]))!
+            let curt = Int(String(numChars[pos..<i + 1]))!
             if pos == 0 {
                 dfs(&res, temp + String(curt), numChars, target, i + 1, curt, curt)
             } else {
diff --git a/DFS/FactorCombinations.swift b/DFS/FactorCombinations.swift
index 18357965..19f7d05c 100644
--- a/DFS/FactorCombinations.swift
+++ b/DFS/FactorCombinations.swift
@@ -28,7 +28,7 @@ class FactorCombinations {
             return
         }
         
-        for i in start ... n where n % i == 0 {
+        for i in start...n where n % i == 0 {
             path.append(i)
             dfs(&res, &path, n / i, i)
             path.removeLast()
diff --git a/DFS/GeneralizedAbbreviation.swift b/DFS/GeneralizedAbbreviation.swift
index c2be3e51..892c4a4c 100644
--- a/DFS/GeneralizedAbbreviation.swift
+++ b/DFS/GeneralizedAbbreviation.swift
@@ -24,7 +24,7 @@ class GeneralizedAbbreviation {
         
         res.append(subset + String(word.count - index))
         
-        for i in index ..< word.count {
+        for i in index..<word.count {
             let offset = i - index
             if offset != 0 {
                 dfs(word, &res, subset + String(offset) + String(word[i]), i + 1)
diff --git a/DFS/LetterCombinationsPhoneNumber.swift b/DFS/LetterCombinationsPhoneNumber.swift
index 4297f0a5..9560b3a8 100644
--- a/DFS/LetterCombinationsPhoneNumber.swift
+++ b/DFS/LetterCombinationsPhoneNumber.swift
@@ -50,7 +50,7 @@ class LetterCombinationsPhoneNumber {
         var temp = temp
         let current = [Character](board[Int(String(chars[index]))!].characters)
         
-        for i in 0 ..< current.count {
+        for i in 0..<current.count {
             temp += String(current[i])
             _dfs(&res, board, chars, temp, index + 1)
             temp = String(temp.characters.dropLast())
diff --git a/DFS/NQueens.swift b/DFS/NQueens.swift
index ad08b658..bd0a5e38 100644
--- a/DFS/NQueens.swift
+++ b/DFS/NQueens.swift
@@ -28,7 +28,7 @@ class NQueens {
         }
         
         
-        for col in 0 ..< n {
+        for col in 0..<n {
             if isValid(board, col, row) {
                 board[row] = setRow(col, n)
                 dfs(&boards, &board, n, row + 1)
@@ -39,8 +39,8 @@ class NQueens {
     private func isValid(_ board: [String], _ col: Int, _ row: Int) -> Bool {
         var c = -1
     
-        for i in 0 ..< row {
-            for j in 0 ..< board[0].characters.count {
+        for i in 0..<row {
+            for j in 0..<board[0].characters.count {
                 if charAt(board[i], j) == "Q" {
                     c = j
                     break
@@ -68,7 +68,7 @@ class NQueens {
     private func setRow(_ col: Int, _ n: Int) -> String {
         var row = ""
         
-        for i in 0 ..< n {
+        for i in 0..<n {
             if i == col {
                 row.append("Q")
             } else {
diff --git a/DFS/NQueensII.swift b/DFS/NQueensII.swift
index 82d8b535..b0218a91 100644
--- a/DFS/NQueensII.swift
+++ b/DFS/NQueensII.swift
@@ -26,7 +26,7 @@
             return
         }
         
-        for col in 0 ..< n {
+        for col in 0..<n {
             if isValid(usedCols, row, col) {
                 usedCols[row] = col
                 dfs(&usedCols, &count, n, row + 1)
@@ -37,7 +37,7 @@
     private func isValid(_ usedCols: [Int], _ row: Int, _ col: Int) -> Bool {
         var c = -1
     
-        for i in 0 ..< row {
+        for i in 0..<row {
             c = usedCols[i] 
             
             // check col
diff --git a/DFS/NumberofIslands.swift b/DFS/NumberofIslands.swift
index e6305298..6b070fd0 100644
--- a/DFS/NumberofIslands.swift
+++ b/DFS/NumberofIslands.swift
@@ -17,8 +17,8 @@ class NumberofIslands {
         let n = grid[0].count
         var count = 0
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if String(grid[i][j]) == "1" {
                     count += 1
                     _dfs(&grid, m, n, i, j)
diff --git a/DFS/PermutationsII.swift b/DFS/PermutationsII.swift
index 49cb705b..da6e7b1f 100644
--- a/DFS/PermutationsII.swift
+++ b/DFS/PermutationsII.swift
@@ -12,7 +12,7 @@ class PermutationsII {
         var path = [Int]()
         var visited = [Bool](count: nums.count, repeatedValue: false)
         
-        let nums = nums.sort({$0 < $1})
+        let nums = nums.sorted(by: <)
         
         _dfs(&res, &path, nums, &visited)
         
@@ -26,7 +26,7 @@ class PermutationsII {
             return
         }
         
-        for i in 0 ..< nums.count {
+        for i in 0..<nums.count {
             if visited[i] || (i > 0 && nums[i] == nums[i - 1] && visited[i - 1]) {
                 continue
             }
diff --git a/DFS/RemoveInvalidParentheses.swift b/DFS/RemoveInvalidParentheses.swift
index 6785baf7..73d11c5e 100644
--- a/DFS/RemoveInvalidParentheses.swift
+++ b/DFS/RemoveInvalidParentheses.swift
@@ -19,7 +19,7 @@ class RemoveInvalidParentheses {
     private func dfs(_ res: inout [String], _ s: [Character], _ lastI: Int, _ lastJ: Int, _ parens: (Character, Character)) {
         var stack = 0, s = s
         
-        for i in lastI ..< s.count {
+        for i in lastI..<s.count {
             if s[i] == parens.0 {
                 stack += 1
             }
@@ -28,9 +28,9 @@ class RemoveInvalidParentheses {
             }
             
             if stack < 0 {
-                for j in lastJ ... i {
+                for j in lastJ...i {
                     if s[j] == parens.1 && (j == lastJ || s[j - 1] != parens.1) {
-                        dfs(&res, Array(s[0 ..< j] + s[j + 1 ..< s.count]), i, j, parens)
+                        dfs(&res, Array(s[0..<j] + s[j + 1..<s.count]), i, j, parens)
                     }
                 }
                 return
diff --git a/DFS/Subsets.swift b/DFS/Subsets.swift
index 031485a3..18f03e53 100644
--- a/DFS/Subsets.swift
+++ b/DFS/Subsets.swift
@@ -11,7 +11,7 @@ class Subsets {
         var res = [[Int]]()
         var path = [Int]()
         
-        let nums = nums.sort({$0 < $1})
+        let nums = nums.sorted(by: <)
         
         _dfs(&res, &path, nums, 0)
         
@@ -23,7 +23,7 @@ class Subsets {
 
         res.append(Array(path))
         
-        for i in index ..< nums.count {
+        for i in index..<nums.count {
             path.append(nums[i])
             _dfs(&res, &path, nums, i + 1)
             path.removeLast()
diff --git a/DFS/SubsetsII.swift b/DFS/SubsetsII.swift
index f521de55..01b42e1f 100644
--- a/DFS/SubsetsII.swift
+++ b/DFS/SubsetsII.swift
@@ -11,7 +11,7 @@ class SubsetsII {
         var res = [[Int]]()
         var path = [Int]()
         
-        let nums = nums.sort({$0 < $1})
+        let nums = nums.sorted(by: <)
         
         _dfs(&res, &path, nums, 0)
         
@@ -21,7 +21,7 @@ class SubsetsII {
     private func _dfs(inout res: [[Int]], inout _ path:[Int], _ nums: [Int], _ index: Int) {
         res.append(Array(path))
         
-        for i in index ..< nums.count {
+        for i in index..<nums.count {
             if i > 0 && nums[i] == nums[i - 1] && i != index {
                 continue
             }
diff --git a/DFS/SudokuSolver.swift b/DFS/SudokuSolver.swift
index d0370d0a..3d969fc1 100644
--- a/DFS/SudokuSolver.swift
+++ b/DFS/SudokuSolver.swift
@@ -17,10 +17,10 @@
     private func helper(_ board: inout [[Character]]) -> Bool {
         let m = board.count, n = board[0].count
     
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if board[i][j] == "." {
-                    for num in 1 ... 9 {
+                    for num in 1...9 {
                         if isValid(board, i, j, Character(String(num))) {
                             board[i][j] = Character(String(num))
                             
@@ -43,22 +43,22 @@
         let m = board.count, n = board[0].count
     
         // check row
-        for x in 0 ..< n {
+        for x in 0..<n {
             if board[i][x] == num {
                 return false
             }
         }
         
         // check col
-        for y in 0 ..< m {
+        for y in 0..<m {
             if board[y][j] == num {
                 return false
             }
         }
         
         // check square
-        for x in i / 3 * 3 ..< i / 3 * 3 + 3 {
-            for y in j / 3 * 3 ..< j / 3 * 3 + 3 {
+        for x in i / 3 * 3..<i / 3 * 3 + 3 {
+            for y in j / 3 * 3..<j / 3 * 3 + 3 {
                 if board[x][y] == num {
                     return false
                 }
diff --git a/DFS/WallsGates.swift b/DFS/WallsGates.swift
index 915dc84f..0f46c309 100644
--- a/DFS/WallsGates.swift
+++ b/DFS/WallsGates.swift
@@ -16,8 +16,8 @@ class WallsGates {
         let m = rooms.count
         let n = rooms[0].count
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if rooms[i][j] == 0 {
                     dfs(&rooms, i, j, m, n, 0)
                 }
diff --git a/DFS/WordSearch.swift b/DFS/WordSearch.swift
index ab72c069..0112132c 100644
--- a/DFS/WordSearch.swift
+++ b/DFS/WordSearch.swift
@@ -17,8 +17,8 @@ class WordSearch {
         var visited = Array(count: m, repeatedValue: Array(count: n, repeatedValue: false))
         var wordContent = [Character](word.characters)
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if board[i][j] == wordContent[0] && _dfs(board, wordContent, m, n, i, j, &visited, 0) {
                     return true
                 }
diff --git a/DFS/WordSearchII.swift b/DFS/WordSearchII.swift
index a9084b82..51aee2d9 100644
--- a/DFS/WordSearchII.swift
+++ b/DFS/WordSearchII.swift
@@ -18,8 +18,8 @@ class WordSearchII {
         let trie = _convertToTrie(words)
         var visited = [[Bool]](count: m, repeatedValue: Array(count: n, repeatedValue: false))
   
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 _dfs(board, m, n, i, j, &visited, &res, trie, "")
             }
         }
@@ -80,7 +80,7 @@ class WordSearchII {
             var node = root
             var word = [Character](word.characters)
     
-            for i in 0 ..< word.count {
+            for i in 0..<word.count {
                 let c = word[i]
       
                 if node.children[c] == nil {
@@ -97,7 +97,7 @@ class WordSearchII {
             var node = root
             var word = [Character](word.characters)
     
-            for i in 0 ..< word.count {
+            for i in 0..<word.count {
                 let c = word[i]
       
                 if node.children[c] == nil {
@@ -114,7 +114,7 @@ class WordSearchII {
             var node = root
             var prefix = [Character](prefix.characters)
     
-            for i in 0 ..< prefix.count {
+            for i in 0..<prefix.count {
                 let c = prefix[i]
       
                 if node.children[c] == nil {
diff --git a/DP/BestTimeBuySellStockCooldown.swift b/DP/BestTimeBuySellStockCooldown.swift
index 8b4de435..3c73e229 100644
--- a/DP/BestTimeBuySellStockCooldown.swift
+++ b/DP/BestTimeBuySellStockCooldown.swift
@@ -14,8 +14,8 @@
         var res = 0
         var dp = Array(repeating: 0, count: prices.count)
         
-        for i in 1 ..< prices.count {
-            for j in (0 ..< i).reversed() {
+        for i in 1..<prices.count {
+            for j in (0..<i).reversed() {
                 if j >= 2 {
                     dp[i] = max(dp[i], prices[i] - prices[j] + dp[j - 2])
                 } else {
diff --git a/DP/BestTimeBuySellStockII.swift b/DP/BestTimeBuySellStockII.swift
index c5826b27..afc0f68e 100644
--- a/DP/BestTimeBuySellStockII.swift
+++ b/DP/BestTimeBuySellStockII.swift
@@ -13,7 +13,7 @@
             return max
         }
         
-        for i in 1 ..< prices.count where prices[i] > prices[i - 1] {
+        for i in 1..<prices.count where prices[i] > prices[i - 1] {
             max += prices[i] - prices[i - 1]
         }
         
diff --git a/DP/BestTimeBuySellStockIII.swift b/DP/BestTimeBuySellStockIII.swift
index ec6c92d6..3a2b32e6 100644
--- a/DP/BestTimeBuySellStockIII.swift
+++ b/DP/BestTimeBuySellStockIII.swift
@@ -26,7 +26,7 @@ class BestTimeBuySellStockIII {
         
         maxProfit = 0
         
-        for i in (0 ..< prices.count).reversed() {
+        for i in (0..<prices.count).reversed() {
             let price = prices[i]
             maxProfit = max(high - price, maxProfit)
             high = max(price, high)
diff --git a/DP/BestTimeBuySellStockIV.swift b/DP/BestTimeBuySellStockIV.swift
index 1bd582c2..1746b23d 100644
--- a/DP/BestTimeBuySellStockIV.swift
+++ b/DP/BestTimeBuySellStockIV.swift
@@ -23,7 +23,7 @@
         
         var local = Array(repeating: 0, count: k + 1), global = Array(repeating: 0, count: k + 1)
         
-        for i in 0 ..< n - 1 {
+        for i in 0..<n - 1 {
             let diff = prices[i + 1] - prices[i]
             for j in stride(from: k, to: 0, by: -1) {
                 local[j] = max(global[j - 1] + max(diff, 0), local[j] + diff)
@@ -37,7 +37,7 @@
     private func makeMaxProfit(_ prices: [Int]) -> Int {
         var sum = 0
     
-        for i in 1 ..< prices.count {
+        for i in 1..<prices.count {
             let diff = prices[i] - prices[i - 1]
             sum += diff > 0 ? diff : 0
         }
diff --git a/DP/BurstBalloons.swift b/DP/BurstBalloons.swift
index d326bc67..6e304fe3 100644
--- a/DP/BurstBalloons.swift
+++ b/DP/BurstBalloons.swift
@@ -18,10 +18,10 @@ class BurstBalloons {
         let n = nums.count, nums = renderNums(nums)
         var dp = Array(repeating: Array(repeating: 0, count: nums.count), count: nums.count)
   
-        for len in 1 ... n {
-            for left in 1 ... n - len + 1 {
+        for len in 1...n {
+            for left in 1...n - len + 1 {
                 let right = left + len - 1
-                    for k in left ... right {
+                    for k in left...right {
                         dp[left][right] = max(dp[left][right], nums[left - 1] * nums[k] * nums[right + 1] + dp[left][k - 1] + dp[k + 1][right])
                     }
             }
diff --git a/DP/CoinChange.swift b/DP/CoinChange.swift
index 7c7bfdbf..24c03fbb 100644
--- a/DP/CoinChange.swift
+++ b/DP/CoinChange.swift
@@ -13,7 +13,7 @@ class CoinChange {
     
         var minNums = [Int](count: amount + 1, repeatedValue: -1)
         
-        for i in 0 ... amount {
+        for i in 0...amount {
             for coin in coins {
                 if coin == i {
                     minNums[i] = 1
diff --git a/DP/CombinationSumIV.swift b/DP/CombinationSumIV.swift
index 13edc72a..1c83e8ba 100644
--- a/DP/CombinationSumIV.swift
+++ b/DP/CombinationSumIV.swift
@@ -13,7 +13,7 @@ class CombinationSumIV {
         dp[0] = 1
         let nums = nums.sorted()
         
-        for i in 1 ... target {
+        for i in 1...target {
             for num in nums {
                 if i < num {
                     break
diff --git a/DP/DecodeWays.swift b/DP/DecodeWays.swift
index dc9430a2..e656e5e5 100644
--- a/DP/DecodeWays.swift
+++ b/DP/DecodeWays.swift
@@ -16,11 +16,11 @@ class DecodeWays {
             return 0
         }
         
-        for i in 1 ... len {
-            if isValid(String(sChars[i - 1 ..< i])) {
+        for i in 1...len {
+            if isValid(String(sChars[i - 1..<i])) {
                 dp[i] += dp[i - 1]
             }
-            if i >= 2 && isValid(String(sChars[i - 2 ..< i])) {
+            if i >= 2 && isValid(String(sChars[i - 2..<i])) {
                 dp[i] += dp[i - 2]
             }
         }
diff --git a/DP/DifferentWaysAddParentheses.swift b/DP/DifferentWaysAddParentheses.swift
index 847f99ce..b196e27c 100644
--- a/DP/DifferentWaysAddParentheses.swift
+++ b/DP/DifferentWaysAddParentheses.swift
@@ -20,8 +20,8 @@ class DifferentWaysAddParentheses {
         
         for (i, char) in chars.enumerated() {
             if char == "+" || char == "*" || char == "-" {
-                let leftResults = diffWaysToCompute(String(chars[0 ..< i]))
-                let rightResults = diffWaysToCompute(String(chars[i + 1 ..< chars.count]))
+                let leftResults = diffWaysToCompute(String(chars[0..<i]))
+                let rightResults = diffWaysToCompute(String(chars[i + 1..<chars.count]))
                 
                 for leftRes in leftResults {
                     for rightRes in rightResults {
diff --git a/DP/EditDistance.swift b/DP/EditDistance.swift
index 1ccb3e63..394860ac 100644
--- a/DP/EditDistance.swift
+++ b/DP/EditDistance.swift
@@ -14,8 +14,8 @@ class EditDistance {
         
         var dp = Array(count: aLen + 1, repeatedValue:(Array(count: bLen + 1, repeatedValue: 0)))
         
-        for i in 0 ... aLen {
-            for j in 0 ... bLen {
+        for i in 0...aLen {
+            for j in 0...bLen {
                 if i == 0 {
                     dp[i][j] = j
                 } else if j == 0 {
diff --git a/DP/FrogJump.swift b/DP/FrogJump.swift
index 48426a74..e5fa20ee 100644
--- a/DP/FrogJump.swift
+++ b/DP/FrogJump.swift
@@ -17,11 +17,11 @@ class FrogJump {
         
         insert(&indexSteps, 0, 0)
         
-        for i in 1 ..< stones.count {
+        for i in 1..<stones.count {
             while maxStep[k] + 1 < stones[i] - stones[k] {
                 k += 1
             }
-            for j in k ..< i {
+            for j in k..<i {
                 let distance = stones[i] - stones[j]
                 
                 if let steps = indexSteps[j], steps.contains(distance - 1) || steps.contains(distance) || steps.contains(distance + 1) {
diff --git a/DP/GenerateParentheses.swift b/DP/GenerateParentheses.swift
index fd491b45..b38d759f 100644
--- a/DP/GenerateParentheses.swift
+++ b/DP/GenerateParentheses.swift
@@ -17,7 +17,7 @@ class GenerateParentheses {
         for str in generateParenthesis(n - 1) {
             for (i, char) in str.characters.enumerated() {
                 if char == "(" {
-                    set.insert(String(Array(str.characters)[0 ... i]) + "()" + String(Array(str.characters)[i + 1 ..< str.characters.count]))
+                    set.insert(String(Array(str.characters)[0...i]) + "()" + String(Array(str.characters)[i + 1..<str.characters.count]))
                 }
             }
             set.insert(str + "()")
diff --git a/DP/GuessNumberHigherOrLowerII.swift b/DP/GuessNumberHigherOrLowerII.swift
index 40b1b214..2e2a8a27 100644
--- a/DP/GuessNumberHigherOrLowerII.swift
+++ b/DP/GuessNumberHigherOrLowerII.swift
@@ -21,7 +21,7 @@ class GuessNumberHigherOrLowerII {
             return table[s][e]
         }
         var guarantee = Int.max
-        for i in s ..< e + 1 {
+        for i in s..<e + 1 {
             // To get the worse case of every choice to guarantee the result
             let tmp = i + max(DP(&table, s, i - 1), DP(&table, i + 1, e))
             // To get the least cost among the guaranteed results
diff --git a/DP/HouseRobberII.swift b/DP/HouseRobberII.swift
index 0bfba9ff..8595e654 100644
--- a/DP/HouseRobberII.swift
+++ b/DP/HouseRobberII.swift
@@ -20,7 +20,7 @@ class HouseRobberII {
     private func _helper(nums:[Int], _ start: Int, _ end: Int) -> Int {
         var pre = 0, cur = 0, res = 0
         
-        for i in start ... end {
+        for i in start...end {
             res = max(pre + nums[i], cur)
             (cur, pre) = (res, cur)
         }
diff --git a/DP/LongestIncreasingSubsequence.swift b/DP/LongestIncreasingSubsequence.swift
index 9072ec85..bb33e11f 100644
--- a/DP/LongestIncreasingSubsequence.swift
+++ b/DP/LongestIncreasingSubsequence.swift
@@ -9,8 +9,8 @@ class LongestIncreasingSubsequence {
         var length_global = 0
         var length_current = [Int](count: nums.count, repeatedValue: 1)
         
-        for i in 0 ..< nums.count {
-            for j in 0 ..< i {
+        for i in 0..<nums.count {
+            for j in 0..<i {
                 if nums[i] > nums[j] {
                     length_current[i] = max(length_current[i], length_current[j] + 1)
                 }
diff --git a/DP/LongestPalindromicSubstring.swift b/DP/LongestPalindromicSubstring.swift
index b22996f7..fa31fd9f 100644
--- a/DP/LongestPalindromicSubstring.swift
+++ b/DP/LongestPalindromicSubstring.swift
@@ -19,12 +19,12 @@ class LongestPalindromicSubstring {
         var isPalin = Array(repeating: Array(repeating: false, count : len), count : len)
         
         // set palindrome whose len is 1
-        for i in 0 ... len - 1 {
+        for i in 0...len - 1 {
             isPalin[i][i] = true
         }
         
         // set palindrome whose len is 2
-        for i in 0 ... len - 2 {
+        for i in 0...len - 2 {
             if sChars[i] == sChars[i + 1] {
                 isPalin[i][i + 1] = true
                 maxLen = 2
@@ -33,8 +33,8 @@ class LongestPalindromicSubstring {
         }
         
         if len >= 3 {
-            for length in 3 ... len {
-                for i in 0 ... len - length {
+            for length in 3...len {
+                for i in 0...len - length {
                     if sChars[i] == sChars[i + length - 1] && isPalin[i + 1][i + length - 2] {
                         isPalin[i][i + length - 1] = true
                         maxLen = length
@@ -44,6 +44,6 @@ class LongestPalindromicSubstring {
             }
         }
         
-        return String(sChars[maxStart ... maxStart + maxLen - 1])
+        return String(sChars[maxStart...maxStart + maxLen - 1])
     }
 }
diff --git a/DP/MaximalSquare.swift b/DP/MaximalSquare.swift
index 631a8f1a..adbd52f8 100644
--- a/DP/MaximalSquare.swift
+++ b/DP/MaximalSquare.swift
@@ -16,8 +16,8 @@ class MaximalSquare {
         var max_global = 0
         var maxSquareSide = Array(count: m, repeatedValue: (Array(count: n, repeatedValue: 0)))
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 guard matrix[i][j] != "0" else {
                     continue;
                 }
diff --git a/DP/MaximumProductSubarray.swift b/DP/MaximumProductSubarray.swift
index 376b971f..95226edc 100644
--- a/DP/MaximumProductSubarray.swift
+++ b/DP/MaximumProductSubarray.swift
@@ -10,7 +10,7 @@ class MaximumProductSubarray {
         var max_soFar = nums[0]
         var max_global = nums[0]
         
-        for i in 1 ..< nums.count {
+        for i in 1..<nums.count {
             var a = max_soFar * nums[i]
             var b = min_soFar * nums[i]
             
diff --git a/DP/MaximumSubarray.swift b/DP/MaximumSubarray.swift
index 64b1149e..571f24bf 100644
--- a/DP/MaximumSubarray.swift
+++ b/DP/MaximumSubarray.swift
@@ -10,7 +10,7 @@ class MaximumSubarray {
         var max_current = nums[0]
         var max_global = nums[0]
         
-        for i in 1 ..< nums.count {
+        for i in 1..<nums.count {
             max_current = max(max_current + nums[i], nums[i])
             max_global = max(max_current, max_global)
         }
diff --git a/DP/MinimumPathSum.swift b/DP/MinimumPathSum.swift
index 77ca44ce..ab455b1a 100644
--- a/DP/MinimumPathSum.swift
+++ b/DP/MinimumPathSum.swift
@@ -13,8 +13,8 @@ class MinimumPathSum {
         let m = grid.count, n = grid[0].count
         var dp = Array(repeating: Array(repeating: 0, count: n), count: m)
         
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if i == 0 && j == 0{
                     dp[i][j] = grid[i][j]
                 } else if i == 0 {
diff --git a/DP/PaintFence.swift b/DP/PaintFence.swift
index 7b5a1ae4..7ee62f4c 100644
--- a/DP/PaintFence.swift
+++ b/DP/PaintFence.swift
@@ -18,7 +18,7 @@ class PaintFence {
         var lastSame = k
         var lastDiff = k * (k - 1)
         
-        for i in 2 ..< n {
+        for i in 2..<n {
             (lastSame, lastDiff) = (lastDiff, (k - 1) * (lastSame + lastDiff))
         }
         
diff --git a/DP/PerfectSquares.swift b/DP/PerfectSquares.swift
index 07a40d08..40317b75 100644
--- a/DP/PerfectSquares.swift
+++ b/DP/PerfectSquares.swift
@@ -14,8 +14,8 @@ class PerfectSquares {
         var leastNums = [Int](count: n + 1, repeatedValue: Int.max)
         leastNums[0] = 0
         
-        for i in 1 ... n {
-            for j in 1 ... i  {
+        for i in 1...n {
+            for j in 1...i  {
                 if j * j > i {
                     break
                 }
diff --git a/DP/Triangle.swift b/DP/Triangle.swift
index 6862999f..c0c87090 100644
--- a/DP/Triangle.swift
+++ b/DP/Triangle.swift
@@ -14,7 +14,7 @@
         var dp = triangle.last!
         
         for i in stride(from: triangle.count - 2, through: 0, by: -1) {
-            for j in 0 ... i {
+            for j in 0...i {
                 dp[j] = min(dp[j], dp[j + 1]) + triangle[i][j]
             }
         }
diff --git a/LinkedList/RemoveNthFromEnd.swift b/LinkedList/RemoveNthFromEnd.swift
index 0b9e7421..80c604b5 100644
--- a/LinkedList/RemoveNthFromEnd.swift
+++ b/LinkedList/RemoveNthFromEnd.swift
@@ -22,7 +22,7 @@ class RemoveNthFromEnd {
         var post: ListNode? = dummy
         
         // move post
-        for _ in 0 ..< n {
+        for _ in 0..<n {
             post = post!.next
         }
         
diff --git a/Math/CountPrimes.swift b/Math/CountPrimes.swift
index 8e7fc547..8951ca46 100644
--- a/Math/CountPrimes.swift
+++ b/Math/CountPrimes.swift
@@ -14,7 +14,7 @@ class CountPrimes {
     
         var isPrime = [Bool](repeating: true, count: n)
         
-        for i in 2 ..< n {
+        for i in 2..<n {
             if isPrime[i] {
                 for j in stride(from: 2 * i, to: n, by: i) {
                     isPrime[j] = false
diff --git a/Math/IntegerBreak.swift b/Math/IntegerBreak.swift
index c1005196..700d3604 100644
--- a/Math/IntegerBreak.swift
+++ b/Math/IntegerBreak.swift
@@ -1,6 +1,6 @@
 /**
  * Question Link: https://leetcode.com/problems/integer-break/
- * Primary idea: Final Result must be split as 2^m * 3^n. Lets say p = p1 + p2 + ... + pn,
+ * Primary idea: Final Result must be split as 2^m * 3^n. Lets say p = p1 + p2 +...+ pn,
  * 				 if p1 could be split as 2(p1 - 2), than it would be greater than p1 if p1 > 4.
  *				 same thing for 3(p1 - 3). Thus we spilt the original number to multiple 3s and 2s to 
  *				 get the final result
diff --git a/Math/IntegerToRoman.swift b/Math/IntegerToRoman.swift
index f113290b..dd292533 100644
--- a/Math/IntegerToRoman.swift
+++ b/Math/IntegerToRoman.swift
@@ -20,7 +20,7 @@
         while number > 0 {
             let current = number / nums[digit]
             
-            for _ in 0 ..< current {
+            for _ in 0..<current {
                 res += symbols[digit]
             }
             
diff --git a/Math/RomanToInteger.swift b/Math/RomanToInteger.swift
index 845edabe..b18b5047 100644
--- a/Math/RomanToInteger.swift
+++ b/Math/RomanToInteger.swift
@@ -11,7 +11,7 @@ class RomanToInteger {
         let chars = [Character](s.characters.reverse())
         var res = 0
         
-        for i in 0 ..< chars.count {
+        for i in 0..<chars.count {
             guard let current = dict[String(chars[i])] else {
                 return res
             }
diff --git a/Math/SingleNumberII.swift b/Math/SingleNumberII.swift
index 878565ca..cc059e07 100644
--- a/Math/SingleNumberII.swift
+++ b/Math/SingleNumberII.swift
@@ -15,10 +15,10 @@ class SingleNumberII {
         var ans = 0
         var sum = 0
         
-        for i in 0 ..< 64 {
+        for i in 0..<64 {
             sum = 0
             let tmp = (1 << i)
-            for j in 0 ..< nums.count {
+            for j in 0..<nums.count {
                 if tmp & nums[j] != 0 {
                     sum = sum + 1
                 }
diff --git a/Math/SudokuSolver.swift b/Math/SudokuSolver.swift
index d0370d0a..3d969fc1 100644
--- a/Math/SudokuSolver.swift
+++ b/Math/SudokuSolver.swift
@@ -17,10 +17,10 @@
     private func helper(_ board: inout [[Character]]) -> Bool {
         let m = board.count, n = board[0].count
     
-        for i in 0 ..< m {
-            for j in 0 ..< n {
+        for i in 0..<m {
+            for j in 0..<n {
                 if board[i][j] == "." {
-                    for num in 1 ... 9 {
+                    for num in 1...9 {
                         if isValid(board, i, j, Character(String(num))) {
                             board[i][j] = Character(String(num))
                             
@@ -43,22 +43,22 @@
         let m = board.count, n = board[0].count
     
         // check row
-        for x in 0 ..< n {
+        for x in 0..<n {
             if board[i][x] == num {
                 return false
             }
         }
         
         // check col
-        for y in 0 ..< m {
+        for y in 0..<m {
             if board[y][j] == num {
                 return false
             }
         }
         
         // check square
-        for x in i / 3 * 3 ..< i / 3 * 3 + 3 {
-            for y in j / 3 * 3 ..< j / 3 * 3 + 3 {
+        for x in i / 3 * 3..<i / 3 * 3 + 3 {
+            for y in j / 3 * 3..<j / 3 * 3 + 3 {
                 if board[x][y] == num {
                     return false
                 }
diff --git a/Math/SuperPow.swift b/Math/SuperPow.swift
index b7b4c4d8..f9772363 100644
--- a/Math/SuperPow.swift
+++ b/Math/SuperPow.swift
@@ -17,7 +17,7 @@ class SuperPow {
 
     private func _pow(a: Int, _ b: Int) -> Int {
       var ret = 1
-      for _ in 0 ..< b {
+      for _ in 0..<b {
         ret = ret * a % base
       }
       return ret
diff --git a/Math/SuperUglyNumber.swift b/Math/SuperUglyNumber.swift
index 47b79f97..b3c55bf4 100644
--- a/Math/SuperUglyNumber.swift
+++ b/Math/SuperUglyNumber.swift
@@ -11,7 +11,7 @@ class SuperUglyNumber {
         var uglyNums = [Int](count: n, repeatedValue: 1)
         var dict = _init(primes)
         
-        for i in 1 ..< n {
+        for i in 1..<n {
             uglyNums[i] = _getMin(&dict, uglyNums, i - 1)
         }
         
diff --git a/Math/TrappingRainWater.swift b/Math/TrappingRainWater.swift
index 7e779c0d..9aa43b30 100644
--- a/Math/TrappingRainWater.swift
+++ b/Math/TrappingRainWater.swift
@@ -16,7 +16,7 @@ class TrappingRainWater {
         var left = _initMaxHeights(height, true)
         var right = _initMaxHeights(height, false)
         
-        for i in 0 ..< height.count {
+        for i in 0..<height.count {
             res += min(left[i], right[i]) - height[i]
         }
         
@@ -28,12 +28,12 @@ class TrappingRainWater {
         var currentMax = 0
         
         if isLeft {
-            for i in 0 ..< height.count {
+            for i in 0..<height.count {
                 res[i] = max(currentMax, height[i])
                 currentMax = res[i]
             }
         } else {
-            for i in (0 ..< height.count).reverse() {
+            for i in (0..<height.count).reverse() {
                 res[i] = max(currentMax, height[i])
                 currentMax = res[i]
             }
diff --git a/Math/UglyNumberII.swift b/Math/UglyNumberII.swift
index ac44a573..e193ca63 100644
--- a/Math/UglyNumberII.swift
+++ b/Math/UglyNumberII.swift
@@ -17,7 +17,7 @@ class UglyNumberII {
         var index3 = 0
         var index5 = 0
         
-        for i in 1 ..< n {
+        for i in 1..<n {
             var minVal = min(uglyNums[index2] * 2, uglyNums[index3] * 3, uglyNums[index5] * 5)
             
             if minVal == uglyNums[index2] * 2 {
diff --git a/Search/MedianTwoSortedArrays.swift b/Search/MedianTwoSortedArrays.swift
index 667f76b1..7531a782 100644
--- a/Search/MedianTwoSortedArrays.swift
+++ b/Search/MedianTwoSortedArrays.swift
@@ -32,9 +32,9 @@ class MedianTwoSortedArrays {
         let j = min(index / 2, n)
         
         if nums1[i - 1] < nums2[j - 1] {
-            return findKth(Array(nums1[i ..< m]), nums2, index - i)
+            return findKth(Array(nums1[i..<m]), nums2, index - i)
         } else {
-            return findKth(nums1, Array(nums2[j ..< n]), index - j)
+            return findKth(nums1, Array(nums2[j..<n]), index - j)
         }
     }
 }
\ No newline at end of file
diff --git a/Sort/AlienDictionary.swift b/Sort/AlienDictionary.swift
index 601b6957..97600fde 100644
--- a/Sort/AlienDictionary.swift
+++ b/Sort/AlienDictionary.swift
@@ -53,7 +53,7 @@ class AlienDictionary {
             }    
         }
         
-        for i in 0 ..< words.count - 1 {
+        for i in 0..<words.count - 1 {
             let prev = Array(words[i].characters)
             let post = Array(words[i + 1].characters)
             var j = 0
diff --git a/Sort/ArrayPartitionI.swift b/Sort/ArrayPartitionI.swift
index 81a69d45..890c841f 100644
--- a/Sort/ArrayPartitionI.swift
+++ b/Sort/ArrayPartitionI.swift
@@ -3,7 +3,7 @@ class ArrayPartitionI {
         var arr = nums
         arr = arr.sorted()
         var res = 0
-        for i in 0 ..< arr.count {
+        for i in 0..<arr.count {
             if i & 1 != 0 {
                 res += min(arr[i], arr[i - 1])
             }
diff --git a/Sort/MeetingRooms.swift b/Sort/MeetingRooms.swift
index 7b750c2a..b8bca988 100644
--- a/Sort/MeetingRooms.swift
+++ b/Sort/MeetingRooms.swift
@@ -28,7 +28,7 @@ class MeetingRooms {
             }
         }
         
-        for i in 0 ..< intervals.count - 1 {
+        for i in 0..<intervals.count - 1 {
             if intervals[i].end > intervals[i + 1].start {
                 return false
             }
diff --git a/Sort/MergeIntervals.swift b/Sort/MergeIntervals.swift
index 06123a5e..bc4cba79 100644
--- a/Sort/MergeIntervals.swift
+++ b/Sort/MergeIntervals.swift
@@ -30,7 +30,7 @@ class MergeIntervals {
 
         var res = [intervals.first!]
         
-        for i in 1 ..< intervals.count {
+        for i in 1..<intervals.count {
             let (last, current) = (res.last!, intervals[i])
             
             if current.start > last.end {
diff --git a/Sort/TopKFrequentElements.swift b/Sort/TopKFrequentElements.swift
index b69793f6..03d68bfd 100644
--- a/Sort/TopKFrequentElements.swift
+++ b/Sort/TopKFrequentElements.swift
@@ -25,6 +25,6 @@ class TopKFrequentElements {
             return value1 > value2
         }
   
-        return Array(keys[0 ..< k])
+        return Array(keys[0..<k])
     }
 }
\ No newline at end of file
diff --git a/Sort/WiggleSortII.swift b/Sort/WiggleSortII.swift
index 45076c5f..9f5b661d 100644
--- a/Sort/WiggleSortII.swift
+++ b/Sort/WiggleSortII.swift
@@ -11,7 +11,7 @@ class WiggleSortII {
         var m = temp.count
         var n = (m + 1) / 2
         
-        for i in 0 ..< nums.count {
+        for i in 0..<nums.count {
             if i & 1 == 0 {
                 n = n - 1
                 nums[i] = temp[n]
diff --git a/Stack/RemoveKDigits.swift b/Stack/RemoveKDigits.swift
index f90fa8ec..b2c02217 100644
--- a/Stack/RemoveKDigits.swift
+++ b/Stack/RemoveKDigits.swift
@@ -18,7 +18,7 @@ class RemoveKDigits {
             res.append(char)
         }
         
-        res = Array(res[0 ..< size])
+        res = Array(res[0..<size])
         for char in res {
             if char != "0" {
                 break
diff --git a/String/CountAndSay.swift b/String/CountAndSay.swift
index 20e56a6a..bdd2da98 100644
--- a/String/CountAndSay.swift
+++ b/String/CountAndSay.swift
@@ -21,13 +21,13 @@ class CountAndSay {
         var chars: [Character]
         var current: Character
   
-        for _ in 1 ..< n{
+        for _ in 1..<n{
             temp = ""
             count = 1
             chars = [Character](res.characters)
             current = chars[0]
     
-            for i in 1 ..< chars.count {
+            for i in 1..<chars.count {
                 if chars[i] == current {
                     count += 1
                 } else {
diff --git a/String/FizzBuzz.swift b/String/FizzBuzz.swift
index f38aefda..952f584d 100644
--- a/String/FizzBuzz.swift
+++ b/String/FizzBuzz.swift
@@ -14,7 +14,7 @@ class FizzBuzz {
             return res
         }
         
-        for i in 1 ... n {
+        for i in 1...n {
             if i % 3 == 0 && i % 5 == 0 {
                 res.append("FizzBuzz")
             } else if i % 3 == 0 {
diff --git a/String/FlipGame.swift b/String/FlipGame.swift
index e4a9e45a..5d162849 100644
--- a/String/FlipGame.swift
+++ b/String/FlipGame.swift
@@ -13,7 +13,7 @@ class FlipGame {
             return res
         }
         
-        for i in 0 ..< sChars.count - 1 {
+        for i in 0..<sChars.count - 1 {
             if sChars[i] == "+" && sChars[i + 1] == "+" {
                 var temp = sChars
                 (temp[i], temp[i + 1])  = ("-", "-")
diff --git a/String/IsomorphicStrings.swift b/String/IsomorphicStrings.swift
index 18705ba5..42e429f3 100644
--- a/String/IsomorphicStrings.swift
+++ b/String/IsomorphicStrings.swift
@@ -16,7 +16,7 @@ class IsomorphicStrings {
             return false
         }
         
-        for i in 0 ..< sChars.count {
+        for i in 0..<sChars.count {
             let sCurrent = sChars[i]
             let tCurrent = tChars[i]
             
diff --git a/String/LengthLastWord.swift b/String/LengthLastWord.swift
index 7c93c9af..7a24d19c 100644
--- a/String/LengthLastWord.swift
+++ b/String/LengthLastWord.swift
@@ -13,7 +13,7 @@ class LengthLastWord {
             return 0
         }
         
-        for i in (0 ... sChars.count - 1).reverse() {
+        for i in (0...sChars.count - 1).reverse() {
             if res == 0 {
                 if sChars[i] == " " {
                     continue
diff --git a/String/LongestCommonPrefix.swift b/String/LongestCommonPrefix.swift
index 29e18d6c..2be7303a 100644
--- a/String/LongestCommonPrefix.swift
+++ b/String/LongestCommonPrefix.swift
@@ -16,12 +16,12 @@ class LongestCommonPrefix {
             var strContent = [Character](str.characters)
             
             if res.count > strContent.count {
-                res = Array(res[0 ..< strContent.count])
+                res = Array(res[0..<strContent.count])
             }
             
-            for i in 0 ..< res.count {
+            for i in 0..<res.count {
                 if res[i] != strContent[i] {
-                    res = Array(res[0 ..< i])
+                    res = Array(res[0..<i])
                     break
                 }
             }
diff --git a/String/LongestSubstringWithoutRepeatingCharacters.swift b/String/LongestSubstringWithoutRepeatingCharacters.swift
index 68ff715e..bb5c623d 100644
--- a/String/LongestSubstringWithoutRepeatingCharacters.swift
+++ b/String/LongestSubstringWithoutRepeatingCharacters.swift
@@ -20,7 +20,7 @@ class LongestSubstringWithoutRepeatingCharacters {
         var startIndex = 0
         var chars = [Character](s.characters)
         
-        for i in 0 ..< chars.count {
+        for i in 0..<chars.count {
             var current = chars[i]
             
             if set.contains(current) {
diff --git a/String/MinimumWindowSubstring.swift b/String/MinimumWindowSubstring.swift
index 7217c16d..de24d788 100644
--- a/String/MinimumWindowSubstring.swift
+++ b/String/MinimumWindowSubstring.swift
@@ -47,7 +47,7 @@ class MinimumWindowSubstring {
                 if freqencyDict[startChar]! > 0 {
                     // update res
                     if i - startIndex + 1 < minLen {
-                        res = String(sChars[startIndex ... i])
+                        res = String(sChars[startIndex...i])
                         minLen = i - startIndex + 1
                     }
                     count -= 1
diff --git a/String/ReverseStringII.swift b/String/ReverseStringII.swift
index 681228be..b223d05d 100644
--- a/String/ReverseStringII.swift
+++ b/String/ReverseStringII.swift
@@ -20,18 +20,18 @@ class ReverseStringII {
             print(i)
     
             if chars.count < i + k {
-                res += chars[i ..< chars.count].reversed()
+                res += chars[i..<chars.count].reversed()
                 break
             }
     
-            res += chars[i ..< i + k].reversed()
+            res += chars[i..<i + k].reversed()
     
             if chars.count < i + 2 * k {
-                res += chars[i + k ..< chars.count]
+                res += chars[i + k..<chars.count]
                 break
             }
     
-            res += chars[i + k ..< i + 2 * k]
+            res += chars[i + k..<i + 2 * k]
         }
   
         return String(res)
diff --git a/String/StrStr.swift b/String/StrStr.swift
index c523dac0..7e0409b3 100644
--- a/String/StrStr.swift
+++ b/String/StrStr.swift
@@ -16,9 +16,9 @@ class StrStr {
             return 0
         }
         
-        for i in 0 ... hLen - nLen {
+        for i in 0...hLen - nLen {
             if hChars[i] == nChars[0] {
-                for j in 0 ..< nLen {
+                for j in 0..<nLen {
                     if hChars[i + j] != nChars[j] {
                         break
                     }
diff --git a/String/TextJustification.swift b/String/TextJustification.swift
index 74765031..b768dfc8 100644
--- a/String/TextJustification.swift
+++ b/String/TextJustification.swift
@@ -14,7 +14,7 @@ class TextJustification {
             return res
         }
         
-        for i in 0 ..< words.count {
+        for i in 0..<words.count {
             let wordLen = words[i].characters.count
         
             if count + (i - last) + wordLen > maxWidth {
@@ -44,10 +44,10 @@ class TextJustification {
         var res = ""
         var extraNum = extraNum
         
-        for i in start ... end {
+        for i in start...end {
             res += words[i]
             if i != end {
-                for _ in 1 ... spaceNum {
+                for _ in 1...spaceNum {
                     res += " "
                 }
                 if extraNum != 0 {
@@ -63,14 +63,14 @@ class TextJustification {
     fileprivate func buildSpecialLine(words: [String], _ start: Int, _ end: Int, _ lineLength: Int) -> String {
         var res = ""
         
-        for i in start ... end {
+        for i in start...end {
             res += words[i]
             if i < end {
                 res += " "
             }
         }
         
-        for _ in res.characters.count ..< lineLength {
+        for _ in res.characters.count..<lineLength {
             res += " "
         }
         
diff --git a/String/WordPattern.swift b/String/WordPattern.swift
index 178dee6b..1c282080 100644
--- a/String/WordPattern.swift
+++ b/String/WordPattern.swift
@@ -15,7 +15,7 @@ class WordPattern {
             return false
         }
         
-        for i in 0 ..< strs.count {
+        for i in 0..<strs.count {
             let currentWord = strs[i]
             let currentChar = patterns[i]
         
diff --git a/String/ZigZagConverstion.swift b/String/ZigZagConverstion.swift
index 66c18293..3503aba5 100644
--- a/String/ZigZagConverstion.swift
+++ b/String/ZigZagConverstion.swift
@@ -19,7 +19,7 @@ class Solution {
         let cnt = chars.count
         
         
-        for i in 0 ..< numRows {
+        for i in 0..<numRows {
             let len = 2 * numRows - 2
             var index = i
             while index < cnt {
diff --git a/Tree/BinaryTreeRightSideView.swift b/Tree/BinaryTreeRightSideView.swift
index ddabdceb..dca72c68 100644
--- a/Tree/BinaryTreeRightSideView.swift
+++ b/Tree/BinaryTreeRightSideView.swift
@@ -28,7 +28,7 @@ class BinaryTreeRightSideView {
         while nodeQ.count > 0 {
             var size = nodeQ.count
             
-            for i in 0 ..< size {
+            for i in 0..<size {
                 let node = nodeQ.removeFirst()
                 
                 if i + 1 == size {
diff --git a/Tree/BinaryTreeVerticalOrderTraversal.swift b/Tree/BinaryTreeVerticalOrderTraversal.swift
index 21f9a1a0..9ffd433a 100644
--- a/Tree/BinaryTreeVerticalOrderTraversal.swift
+++ b/Tree/BinaryTreeVerticalOrderTraversal.swift
@@ -52,7 +52,7 @@ class BinaryTreeVerticalOrderTraversal {
             }
         }
         
-        for col in minCol ... maxCol {
+        for col in minCol...maxCol {
             res.append(colVals[col]!)
         }
         
diff --git a/Tree/ConstructBinaryTreeInorderPostorder.swift b/Tree/ConstructBinaryTreeInorderPostorder.swift
index 88545a7a..2e112757 100644
--- a/Tree/ConstructBinaryTreeInorderPostorder.swift
+++ b/Tree/ConstructBinaryTreeInorderPostorder.swift
@@ -34,7 +34,7 @@ class ConstructBinaryTreeInorderPostorder {
         let root = TreeNode(postorder[postEnd])
         
         var mid = 0
-        for i in inStart ... inEnd where inorder[i] == root.val {
+        for i in inStart...inEnd where inorder[i] == root.val {
             mid = i
             break
         }
diff --git a/Tree/ConstructBinaryTreePreorderInorder.swift b/Tree/ConstructBinaryTreePreorderInorder.swift
index 29ec598d..60fd418f 100644
--- a/Tree/ConstructBinaryTreePreorderInorder.swift
+++ b/Tree/ConstructBinaryTreePreorderInorder.swift
@@ -34,7 +34,7 @@ class ConstructBinaryTreePreorderInorder {
         let root = TreeNode(preorder[preStart])
         
         var mid = 0
-        for i in inStart ... inEnd {
+        for i in inStart...inEnd {
             if inorder[i] == preorder[preStart] {
                 mid = i
                 break
diff --git a/Tree/UniqueBinarySearchTrees.swift b/Tree/UniqueBinarySearchTrees.swift
index 38ea43e8..ce3eb5d3 100644
--- a/Tree/UniqueBinarySearchTrees.swift
+++ b/Tree/UniqueBinarySearchTrees.swift
@@ -26,8 +26,8 @@ class UniqueBinarySearchTrees {
         var dp = Array(repeating: 0, count: n + 1)
         dp[0] = 1
         
-        for i in 1 ... n {
-            for j in 0 ..< i {
+        for i in 1...n {
+            for j in 0..<i {
                 dp[i] += dp[j] * dp[i - j - 1]
             }
         }
diff --git a/UnionFind/GraphValidTree.swift b/UnionFind/GraphValidTree.swift
index 4f929570..12efe02c 100644
--- a/UnionFind/GraphValidTree.swift
+++ b/UnionFind/GraphValidTree.swift
@@ -13,7 +13,7 @@ class GraphValidTree {
             return true
         }
         
-        var roots = [Int](0 ..< n)
+        var roots = [Int](0..<n)
         var count = n
         
         for edge in edges {
diff --git a/UnionFind/NumberConnectedComponentsUndirectedGraph.swift b/UnionFind/NumberConnectedComponentsUndirectedGraph.swift
index b830fbb5..8d40b5d2 100644
--- a/UnionFind/NumberConnectedComponentsUndirectedGraph.swift
+++ b/UnionFind/NumberConnectedComponentsUndirectedGraph.swift
@@ -13,7 +13,7 @@ class NumberConnectedComponentsUndirectedGraph {
         }
     
         var count = n
-        var roots = [Int](0 ... n - 1)
+        var roots = [Int](0...n - 1)
         
         for edge in edges {
             let root0 = findRoot(edge[0], roots)