一. 题目描述

Invert a binary tree.

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

to

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

Trivia:

This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

二. 题目分析

题目意图很明显，即翻转一棵二叉树。后面是几句话，大概的意思是：

Google：我们有90%的工程师在使用你写的软件（Homebrew？），但你居然不会在白板上翻转一棵二叉树，真是操蛋。

这是一道任何程序员都应该会的题，递归或者队列迭代解法都可以实现，不多加赘述。

三. 示例代码

// C++，递归
/** * 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 == NULL) return root;
        TreeNode* temp = root->left;
        root->left = root->right;
        root->right = temp;

        invertTree(root->left);
        invertTree(root->right);

        return root;
    }
};

# Python，递归

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

class Solution(object):
    def invertTree(self, root):
        """ :type root: TreeNode :rtype: TreeNode """
        if root is None:
            return None
        root.left, root.right = root.right, root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root

四. 小结

该题意在帮助我们复习数据结构的基础。