[Math] add Solution to Plus One and Ugly Number II

Math/PlusOne.swift

@@ -0,0 +1,26 @@
+/**
+ * Question Link: https://leetcode.com/problems/plus-one/
+ * Primary idea: Iterate and change the array from last to the first
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ */
+
+class PlusOne {
+    func plusOne(digits: [Int]) -> [Int] {
+        var digits = digits
+        var index = digits.count - 1
+
+        while index >= 0 {
+            if digits[index] < 9 {
+                digits[index] += 1
+                return digits
+            }
+
+            digits[index] = 0
+            index -= 1
+        }
+
+        digits.insert(1, atIndex: 0)
+        return digits
+    }
+}

Math/UglyNumberII.swift

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+/**
+ * Question Link: https://leetcode.com/problems/ugly-number-ii/
+ * Primary idea: Use three indices to build a helper array
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ */
+
+class UglyNumberII {
+    func nthUglyNumber(n: Int) -> Int {
+        guard n > 1 else {
+            return 1
+        }
+
+        var uglyNums = [Int](count: n, repeatedValue: 1)
+
+        var index2 = 0
+        var index3 = 0
+        var index5 = 0
+
+        for i in 1 ..< n {
+            var minVal = min(uglyNums[index2] * 2, uglyNums[index3] * 3, uglyNums[index5] * 5)
+
+            if minVal == uglyNums[index2] * 2 {
+                index2 += 1
+            } 
+            if minVal == uglyNums[index3] * 3 {
+                index3 += 1
+            } 
+            if minVal == uglyNums[index5] * 5{
+                index5 += 1
+            }
+
+            uglyNums[i] = minVal
+        }
+
+        return uglyNums[n - 1]
+    }
+}