2、下面是设计链表类，请按要求补充成员函数中的空。
class CList;
 class CItem
{  public:
     friend class CList;
   private:
     CItem(int d1,int d2){x=d1;y=d2; next=NULL;}
     unsigned int x,y;
     CItem *next;
};
class CList
{  public:
     CList(){head=NULL;}
     CItem * append(unsigned int d1,unsigned int d2);
     ~CList();
  private:
     CItem *end();
     CItem *head;
};
CItem * CList::end()//返回链表的最后一个节点的地址
{  CItem *prev,*pt;
   Prev=pt=head ;
   while (pt)
{  prv=pt ;
    __________________________//下一个节点
}
__________________________
}
CItem * CList::append(unsigned int d1,unsigned int d2)
{//将新的节点添加到链表的前端
   CItem  *pt = ___________________//产生新的节点并且会成功
   if ( head ==NULL )
       ________________________________//这两句把节点添加到
   else
    _________________________________//链表末尾
    return  pt ;
}
CList::~CList( )
{   CItem *pt;
    pt=head;
   while (pt)
{
   head=pt->next;
    ___________________________
    ___________________________
}
}