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