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.

/* 求翻轉二叉樹，其實就是求二叉樹的鏡像


*/

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

Leetcode 226. Invert Binary Tree(easy)