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 ``` [leetcode](https://leetcode-cn.com/problems/invert-binary-tree)