/**

* 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 *tmp = root->left;

        root->left = root->right;

        root->right = tmp;

        invertTree(root->left);

        invertTree(root->right);

        return root;

    }

};