s->next = p;
p->prev = s;
q->next = s;
s->prev = q;

s->prev = p;
s->next = p->next;
p->next->prev = s;
p->next = s;

s->next = p;
s->prev = p->prev;
p->prev->next = s;
p->prev = s;

s->next = p;
s->prev = nullptr;
p->prev = s;