添加 101. 对称二叉树 Swift版本

problems/0101.对称二叉树.md

@@ -574,7 +574,87 @@ var isSymmetric = function(root) {
 };
 ```
 
+## Swift:
 
+> 递归
+```swift
+func isSymmetric(_ root: TreeNode?) -> Bool {
+    return _isSymmetric(root?.left, right: root?.right)
+}
+func _isSymmetric(_ left: TreeNode?, right: TreeNode?) -> Bool {
+    // 首先排除空节点情况
+    if left == nil && right == nil {
+        return true
+    } else if left == nil && right != nil {
+        return false
+    } else if left != nil && right == nil {
+        return false
+    } else if left!.val != right!.val {
+        // 进而排除数值不相等的情况
+        return false
+    }
+    
+    // left 和 right 都不为空, 且数值也相等就递归
+    let inSide = _isSymmetric(left!.right, right: right!.left)
+    let outSide = _isSymmetric(left!.left, right: right!.right)
+    return inSide && outSide
+}
+```
+
+> 迭代 - 使用队列
+```swift
+func isSymmetric2(_ root: TreeNode?) -> Bool {
+    guard let root = root else {
+        return true
+    }
+    var queue = [TreeNode?]()
+    queue.append(root.left)
+    queue.append(root.right)
+    while !queue.isEmpty {
+        let left = queue.removeFirst()
+        let right = queue.removeFirst()
+        if left == nil && right == nil {
+            continue
+        }
+        if left == nil || right == nil || left?.val != right?.val {
+            return false
+        }
+        queue.append(left!.left)
+        queue.append(right!.right)
+        queue.append(left!.right)
+        queue.append(right!.left)
+    }
+    return true
+}
+```
+
+> 迭代 - 使用栈
+```swift
+func isSymmetric3(_ root: TreeNode?) -> Bool {
+    guard let root = root else {
+        return true
+    }
+    var stack = [TreeNode?]()
+    stack.append(root.left)
+    stack.append(root.right)
+    while !stack.isEmpty {
+        let left = stack.removeLast()
+        let right = stack.removeLast()
+        
+        if left == nil && right == nil {
+            continue
+        }
+        if left == nil || right == nil || left?.val != right?.val {
+            return false
+        }
+        stack.append(left!.left)
+        stack.append(right!.right)
+        stack.append(left!.right)
+        stack.append(right!.left)
+    }
+    return true
+}
+```
 
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>