A
q-> next=p；p -> next=q；q-> prior ->next= q；q-> next=p；
B
q ->prior= p -> prior；p -> prior ->next=q；q-> next=p；p -> prior = q->next；
C
p -> prior ->next=q；q-> next=p；q -> prior = p->prior；p -> prior = q；
D
p -> prior = q；q-> next=p；p -> prior ->next=q； q ->prior= p -> prior；
