通过（ ）操作，能完成在双向循环链表结点 p 之后插入结点 s 的功能（其中 next 域为结点的直接后继， prev 域为结点的直接前驱）。
p->next->prev = s; s->prev = p; p->next = s; s->next = p->next;
p->next->prev = s; p->next = s; s->prev = p; s->next = p->next;
s->prev = p; s->next = p->next; p->next = s; p->next->prev = s;
s->next = p->next; p->next->prev = s; s->prev = p; p->next = s;