Flatten a Multilevel Doubly Linked List

 Node* flatten(Node* head) {

        Node *cur = head;

        while (cur)
        {
            if (cur -> child == nullptr)
            {
                cur = cur -> next ;
                continue;
            }

            Node *end_cur_child = cur -> child;

            while (end_cur_child -> next != nullptr)
            {
                end_cur_child = end_cur_child -> next;
            }


            Node *save_next = cur -> next;
            cur -> next = cur -> child;
            cur -> child -> prev = cur;
            end_cur_child -> next = save_next;
            if (save_next)
            save_next -> prev = end_cur_child;
            cur -> child = nullptr;

             cur = cur -> next ;

        }
        return head;
    }