A.rear-front+m)%m B.rear-front+1 C.rear-front-1 D.rear-front
A.1,2,3,4,5,6 B.3,2,6,4,5,1 C.2,4,6,5,3,1 D.6,5,4,3,2,1
A.t->next=r;t=r; B.r->next=s;s=r; C.s->next=r;s=r; D.r->next=t;