feat: add swift implementation to lcof problem: No.28 (doocs#2895)

Author: klever34

lcof/面试题28. 对称的二叉树/README.md

@@ -308,6 +308,38 @@ 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 isSymmetric(_ root: TreeNode?) -> Bool {
+        return dfs(root, root)
+    }
+
+    private func dfs(_ a: TreeNode?, _ b: TreeNode?) -> Bool {
+        if a == nil && b == nil {
+            return true
+        }
+        if a == nil || b == nil || a!.val != b!.val {
+            return false
+        }
+        return dfs(a!.left, b!.right) && dfs(a!.right, b!.left)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题28. 对称的二叉树/Solution.swift

@@ -0,0 +1,27 @@
+/* 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 isSymmetric(_ root: TreeNode?) -> Bool {
+        return dfs(root, root)
+    }
+
+    private func dfs(_ a: TreeNode?, _ b: TreeNode?) -> Bool {
+        if a == nil && b == nil {
+            return true
+        }
+        if a == nil || b == nil || a!.val != b!.val {
+            return false
+        }
+        return dfs(a!.left, b!.right) && dfs(a!.right, b!.left)
+    }
+}