周练（2）226. 翻转二叉树

226. 翻转二叉树

翻转一棵二叉树。

示例：

输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9
输出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

C++版本：

/*
 * @lc app=leetcode.cn id=226 lang=cpp
 *
 * [226] 翻转二叉树
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr) {
            return root;
        }

        invertTree(root->left);
        invertTree(root->right);
        if (root) {
            TreeNode *t = root->left;
            root->left = root->right;
            root->right = t;
        }
        return root;
    }
};
// @lc code=end

python版本：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        if root == None:
            return root
        self.invertTree(root.left)
        self.invertTree(root.right)
        if root:
            t = root.left
            root.left = root.right
            root.right = t 
        return root