题目：

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.

提示：

此题考察函数递归的应用及swap操作。

代码：

/**
 * 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) return NULL;
        // 保存左侧节点
        TreeNode* node = root->left;
        // 对右侧节点进行递归逆序，然后赋给left指针
        root->left = invertTree(root->right);
        // 对之前保存的左侧节点进行递归，赋给右侧节点
        root->right =  invertTree(node);
        return root;
    }
};