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.O(1) B.O(n) C.O(n2) D.O(log2n)