[leetcode] Invert Binary Tree

From : https://leetcode.com/problems/invert-binary-tree/

nvert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9

to

     4
   /   \
  7     2
 / \   / \
9   6 3   1

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