bool helper(TreeNode *pA, TreeNode *pB) {        if (!pA && !pB) return true;        if (!pA || !pB) return false;  // only one has node in a tree and b tree        if (pA->val != pB->val) return false;        return helper(pA->left, pB->.right) && helper(pA->right, pB->left); // a goes in in-order traversal, b goes right first then left.    }    public:    bool isSymmetric(TreeNode *root) {        if (!root)            return true;                return helper(root->left, root->right);    }