p->prior = s;s->next = p;p->prior->next = s;s->prior = p->prior
p->prior = s;p->prior->next = s;s->next = p;s->prior = p->prior
s->next = p;s->prior = p->prior;p->prior = s;p->prior->next = s
s->next = p;s->prior = p->prior;p->prior->next = s;p->prior = s