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 == NULL)
    return NULL;
    else
    {TreeNode *temp =root->right;
    root->right = invertTree(root->left);
    root->left = invertTree(temp);
    return root;
    }
    
       
    }
};