bool check(TreeNode* root) {
    if (!root) return true;

    queue<TreeNode*> q;
    q.push(root);

    bool hasNull = false;

    while (!q.empty()) {
        TreeNode* cur = q.front();
        q.pop();

        if (cur == nullptr) {
            hasNull = true;
        } else {
            if (hasNull) return false;
            q.push(cur->left);
            q.push(cur->right);
        }
    }

    return true;
}