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