```
class Solution {
public:
bool helper(TreeNode *l, TreeNode *r){
//if r and l equal NULL, they equal each other,
//it is risky but cann't pass
if(r == l) return true;
if(l == NULL || r == NULL) return false;
if(l->val != r->val) return false;
return helper(l->left, r->right) && helper(l->right, r->left);
}
bool isSymmetric(TreeNode *root) {
if(root == NULL) return true;
else return helper(root->left, root->right);
}
};
```

it can reach 13ms. When r and l equal NULL, they are the same, which is the same as r == NULL && l == NULL