feat: add swift implementation to lcof problem: No.27 (doocs#2894)

Author: klever34

lcof/面试题27. 二叉树的镜像/README.md

@@ -281,6 +281,36 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+/* public class TreeNode {
+*     var val: Int
+*     var left: TreeNode?
+*     var right: TreeNode?
+*     init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    func mirrorTree(_ root: TreeNode?) -> TreeNode? {
+        guard let root = root else {
+            return nil
+        }
+        let temp = root.left
+        root.left = root.right
+        root.right = temp
+        _ = mirrorTree(root.left)
+        _ = mirrorTree(root.right)
+        return root
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题27. 二叉树的镜像/Solution.swift

@@ -0,0 +1,25 @@
+/* public class TreeNode {
+*     var val: Int
+*     var left: TreeNode?
+*     var right: TreeNode?
+*     init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    func mirrorTree(_ root: TreeNode?) -> TreeNode? {
+        guard let root = root else {
+            return nil
+        }
+        let temp = root.left
+        root.left = root.right
+        root.right = temp
+        _ = mirrorTree(root.left)
+        _ = mirrorTree(root.right)
+        return root
+    }
+}