bool isCompleteTree(TreeNode* root) {
  if (root == nullptr) return true;
  queue<TreeNode*> q;
  q.push(root);
  bool hasNull = false;
  while (!q.empty()) {
    TreeNode* node = q.front();
    q.pop();
    if (node == nullptr) {
      hasNull = true;
    } else {
      if (hasNull) return false;
      q.push(node->left);
      q.push(node->right);
    }
  }
  return true;
}