针对前插:
s->prior = p->prior;
p->prior->pnext = s;
s->pnext = p;
p->prior = s;
针对后插:
s->pnext = p->pnext;
p->pnext->prior = s;
s->prior = p;
p->pnext = s;
/* 插入p的前面 */
int*q;
q=p->prior;
s->next=p;
s->prior=q;
q->next=s;
p->prior=s;
/* 插入p的后面 */
int*q;
q=p->next;
s->next=q;
s->prior=p;
q->prior=s;
p->next=s;