Ap->prior = s；s->next = p；p->prior->next = s；s->prior = p->prior
Bp->prior = s；p->prior->next = s；s->next = p；s->prior = p->prior
Cs->next = p；s->prior = p->prior；p->prior = s；p->prior->next = s
Ds->next = p；s->prior = p->prior；p->prior->next = s；p->prior = s