solve

Author: sonjh1217

remove-nth-node-from-end-of-list/sonjh1217.swift

@@ -0,0 +1,50 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init() { self.val = 0; self.next = nil; }
+ *     public init(_ val: Int) { self.val = val; self.next = nil; }
+ *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+class Solution {
+    // O(n) time / O(n) space
+    func removeNthFromEndList(_ head: ListNode?, _ n: Int) -> ListNode? {
+        var nodes = [ListNode]()
+        var node = head
+        while let current = node {
+            nodes.append(current)
+            node = current.next
+        }
+        
+        let indexToRemove = nodes.count - n
+        
+        if indexToRemove == 0 {
+            return head?.next
+        }
+        
+        nodes[indexToRemove - 1].next = indexToRemove + 1 < nodes.count ? nodes[indexToRemove + 1] : nil
+        return head
+    }
+    
+    // O(n) time / O(1) space
+    func removeNthFromEndPointer(_ head: ListNode?, _ n: Int) -> ListNode? {
+        let dummy = ListNode(0)
+        dummy.next = head
+        var post: ListNode? = dummy
+        var prev: ListNode? = dummy
+
+        for _ in 0...n {
+            post = post?.next
+        }
+
+        while post != nil {
+            prev = prev?.next
+            post = post?.next
+        }
+
+        prev?.next = prev?.next?.next
+        return dummy.next
+    }
+}