leetcode筆記：Invert Binary Tree

一. 題目描述

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. 題目分析

題目意圖很明顯，即翻轉一棵二叉樹。後面是幾句話，大概的意思是：

Google：我們有90%的工程師在使用你寫的軟件（Homebrew？），但你居然不會在白板上翻轉一棵二叉樹，真是操蛋。

這是一道任何程序員都應該會的題，遞歸或者隊列迭代解法都可以實現，不多加贅述。

三. 示例代碼

// C++，遞歸
/**
 * 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，遞歸

# 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. 小結

該題意在幫助我們復習數據結構的基礎。