方法一

L *link(L *head)
{
      L *p, *q;
      p = head->next;
      head - >next = NULL;
      while ( q ! = NULL)
      {
                    q = p->next;
                    p = head->next;
                    head->next = p;
                    p = q;
       }
      return head;
}  

将原先的链表节点摘取下来，使用头结点插入的方法再插入，最后实现单链表的逆序

方法二

L *link(L *head)
{
      L *pre , *p = head->next, *r = p->next;
      p->next = NULL;
      while (r != NULL)
      {
            pre = p;
            p=r;
            r = r->next;
            p->next = pre; 
      }
      head->next = p
      return head;
}