LeetCode-226 Invert Binary Tree

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

Invert 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 == NULL)
            return NULL;
        TreeNode *ptr = root->left;
        root->left = root->right;
        root->right = ptr;
        invertTree(root->left);
        invertTree(root->right);
        return root;
    }
};