if (!root) return true;

queue<TreeNode*> queue;

queue.push(root->left);

queue.push(root->right);

while (!queue.empty()) {

TreeNode* left = queue.front(); queue.pop();

TreeNode* right = queue.front(); queue.pop();

if (!left && !right) continue;

if (!left || !right) return false;

if (left->val != right->val) return false;

queue.push(left->left);

queue.push(right->right);

queue.push(left->right);

queue.push(right->left);

}

return true;

if (left == nullptr && right == nullptr) return true;

if (left != nullptr && right != nullptr) {

return left->val == right->val && isSymmetric(left->left, right->right) && isSymmetric(left->right, right->left);

}

else return false;