Invert Binary Tree

/**
* 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* l = invertTree(root->left);
TreeNode* r = invertTree(root->right);
root->left = r;
root->right = l;
return root;
}
};