/**
 * 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 0;

        if(root->left == NULL && root->right == NULL)
            return root;

        root->left = invertTree(root->left);
        root->right = invertTree(root->right);

        swap(root->left, root->right);

        return root;
    }
};