A.f-〉next=c；f=s
B.r-〉next=s；r=s
C.s-〉next=r；r=s
D.s-〉next=f；f=s

A.(sq.rear+1)%maxsize==(sq.front+1)%maxsize
B.(sq.rear+1)%maxsize==sq.front+1
C.sq.(rear+1)%maxsize==sq.front
D.sq.rear==sq.front

A.sq.front=(sq.front+1)%maxsize
B.sq.front=sq.front+1
C.sq.rear=(sq.rear+1)%maxsize
D.sq.rear=sq.rear+1

A.top不变
B.top=0
C.top--
D.top++