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.

反转一下二叉树。

首先看完题直接能写出A过的都应该骄傲一下。因为Homebrew的作者竟然没写出来。刚开始我是用BFS来写，比较复杂。

递归的方式