[LinkedList] add solution to Remove Nth Node From End of List

LinkedList/RemoveNthFromEnd.swift

@@ -0,0 +1,47 @@
+/**
+ * Question Link: https://leetcode.com/problems/remove-nth-node-from-end-of-list/
+ * Primary idea: Runner Tech
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.next = nil
+ *     }
+ * }
+ */
+
+class RemoveNthFromEnd {
+    func removeNthFromEnd(head: ListNode?, _ n: Int) -> ListNode? {
+        guard let head = head else {
+            return nil
+        }
+
+        let dummy = ListNode(0)
+        dummy.next = head
+        var prev: ListNode? = dummy
+        var post: ListNode? = dummy
+
+        // move post
+        for _ in 0 ..< n {
+            if post == nil {
+                break
+            }
+
+            post = post!.next
+        }
+
+        // move prev and post at the same time
+        while post != nil && post!.next != nil {
+            prev = prev!.next
+            post = post!.next
+        }
+
+        prev!.next = prev!.next!.next
+
+        return dummy.next
+    }
+}