A.hs->next=s; B.s->next=hs;hs=s; C.s->next=hs->next;hs->next=s; D.s->next=hs;hs=hs->next;
A.top不變 B.top=0 C.top-- D.top++
A.5,4,3,2,1 B.2,1 C.2,3 D.3,4