A.n B.n/2 C.(n-1)/2 D.(n+1)/2
A.s=rear;rear=rear->link;deletes B.rear=rear->link;deleterear C.rear=rear->link->link;deleterear D.s=rear->link->link;rear->link->link=s->link;deletes
A.p->link==NULL B.p==NULL C.p->link==first D.p==first