void *del(student *head, long num)
 {
    student *p1,*p2;
    if(head==NULL)
    {return (head);}
    p1=head;
    while(num!=p1->num && p1->next !=NULL)
    {
        p2=p1;
        p1=p1->next;
    }
    if(num == p1->num)
    {
        if(p1==head)
                head=p1->next;
        else
                p2->next=p1->next;
        cout<<"delete:"<<num<<endl;
        n=n-1;
    }
    else
        cout<<"can not find"<<num;
    return(head);
 }