leetcode 226. Invert Binary Tree

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        
        TreeNode *l,*r;
        if(root==NULL)return root;
        l = root->left;
        r = root->right;
        root->left = invertTree(r);
        root->right = invertTree(l);
        return root;
        
    }
};