A.p=p->next; B.p->next=p->next->next; C.p->next=p; D.p=p->next->next;
A.s->next=p->next;p->next=s; B.p->next=s->next;s->next=p; C.q->next=s;s->next=p; D.p->next=s;s->next=q;
A.(n-1)/2 B.n/2 C.(n+1)/2 D.n