A.p->next=s;p->next=s->next B.p->next=s;s->next=p->next C.p->next=s->next;p->next=s D.s->next=p->next;p->next=s
A.p!=L->first B.p==L->first C.p!=NULL D.p->next!=NULL
A.head->next==head B.head!=NULL C.head==NULL D.head->next==NULL