struct student * del(struct student * head, long del_num)
{
    struct student * p1, * p2;
    p1 = p2 = head;
    if(head == NULL)    
    {
        printf("\n链表为空!\n");
        goto end;
    }
        while(p1 -> num != del_num && p1 -> next != NULL);//能不能在这用else改呢？？？？
        {
            p2 = p1;
            p1 = p2 -> next;
        }
            if(p1 -> num == del_num)
            {
                if(head == p1)   head = p1 -> next;
                else
                    p2 -> next = p1 -> next;
                n = n - 1;
                printf("已删除%ld!", del_num);
            }
            else
                printf("无法找到%d元素!", del_num);    
               end;
        return (head);
}