feat: add swift implementation to lcof2 problem: No.021 (#3009)

Author: klever34

lcof2/剑指 Offer II 021. 删除链表的倒数第 n 个结点/README.md

@@ -238,6 +238,41 @@ def remove_nth_from_end(head, n)
 end
 ```
 
+#### Swift
+
+```swift
+/* class ListNode {
+*     var val: Int
+*     var next: ListNode?
+*     init() { self.val = 0; self.next = nil }
+*     init(_ val: Int) { self.val = val; self.next = nil }
+*     init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next }
+* }
+*/
+
+class Solution {
+    func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
+        let dummy = ListNode(0, head)
+        var fast: ListNode? = dummy
+        var slow: ListNode? = dummy
+        
+        var n = n
+        while n > 0 {
+            fast = fast?.next
+            n -= 1
+        }
+        
+        while fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next
+        }
+        
+        slow?.next = slow?.next?.next
+        return dummy.next
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 021. 删除链表的倒数第 n 个结点/Solution.swift

@@ -0,0 +1,30 @@
+/* class ListNode {
+*     var val: Int
+*     var next: ListNode?
+*     init() { self.val = 0; self.next = nil }
+*     init(_ val: Int) { self.val = val; self.next = nil }
+*     init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next }
+* }
+*/
+
+class Solution {
+    func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
+        let dummy = ListNode(0, head)
+        var fast: ListNode? = dummy
+        var slow: ListNode? = dummy
+        
+        var n = n
+        while n > 0 {
+            fast = fast?.next
+            n -= 1
+        }
+        
+        while fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next
+        }
+        
+        slow?.next = slow?.next?.next
+        return dummy.next
+    }
+}