bool isSymmetric(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(!root)            return true;        return helper(root->left,root->right);    }        bool helper(TreeNode* root1, TreeNode* root2)    {        if(!root1&&!root2)            return true;        if(!root1||!root2)            return false;        if(root1->val!=root2->val)            return false;        return helper(root1->left,root2->right)&&helper(root1->right,root2->left);    }