【Invert Binary Tree】cpp

题目：

Invert Binary Tree

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Question Solution 

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.

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 root;
            TreeNode* tmp = root->left;
            root->left = Solution::invertTree(root->right);
            root->right = Solution::invertTree(tmp);
            return root;
    }
};

tips：

翻转二叉树跟交换两个int差不多。。。重点是留一个tmp。