feat: add swift implementation to lcof problem: No.55.2 (doocs#2942)

Author: klever34

lcof/面试题55 - II. 平衡二叉树/README.md

@@ -309,6 +309,40 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+/* public class TreeNode {
+*     public var val: Int
+*     public var left: TreeNode?
+*     public var right: TreeNode?
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    func isBalanced(_ root: TreeNode?) -> Bool {
+        return dfs(root) != -1
+    }
+
+    private func dfs(_ root: TreeNode?) -> Int {
+        guard let root = root else {
+            return 0
+        }
+        let leftDepth = dfs(root.left)
+        let rightDepth = dfs(root.right)
+        if leftDepth == -1 || rightDepth == -1 || abs(leftDepth - rightDepth) > 1 {
+            return -1
+        }
+        return 1 + max(leftDepth, rightDepth)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题55 - II. 平衡二叉树/Solution.swift

@@ -0,0 +1,29 @@
+/* public class TreeNode {
+*     public var val: Int
+*     public var left: TreeNode?
+*     public var right: TreeNode?
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    func isBalanced(_ root: TreeNode?) -> Bool {
+        return dfs(root) != -1
+    }
+
+    private func dfs(_ root: TreeNode?) -> Int {
+        guard let root = root else {
+            return 0
+        }
+        let leftDepth = dfs(root.left)
+        let rightDepth = dfs(root.right)
+        if leftDepth == -1 || rightDepth == -1 || abs(leftDepth - rightDepth) > 1 {
+            return -1
+        }
+        return 1 + max(leftDepth, rightDepth)
+    }
+}