循环链表 无法析构的问题

#include<iostream>
class ListEntry                                       //链表的结点
{
 private:
    double coef;
    int expn;
    ListEntry* preventry;
    ListEntry* nextentry;
 public:
    ListEntry(double a_coef=0,int a_expn=0);
    ~ListEntry(){};
    friend class List;
};
 ListEntry::ListEntry(double a_coef,int a_expn)
{   
    coef=a_coef;
    expn=a_expn;
    preventry=0;
    nextentry=0;
}
class List
{
private:
    ListEntry* head;
    int length;
public:
    List();
    ~List(){};
    void Delete(int i);
    void InsFirst(ListEntry* q);

};

List::List()
{
    head=new ListEntry;
    head->preventry=head->nextentry=head;   
    length=0;

}

void List::InsFirst(ListEntry* q)
{
   q->preventry=head;
   q->nextentry=head->nextentry;
   head->nextentry->preventry=q;
   head->nextentry=q;
   length++;

  return;
}

void List::Delete(int i)
{
    if(i>0 && i<=length)
    {
        ListEntry* p=head->nextentry;
        while( --i)
        {
            p=p->nextentry;
        }
         p->preventry->nextentry=p->nextentry;
         p->nextentry->preventry=p->preventry;
         p=0;
         delete p;
         length--;

    }
    else
        std::cerr<<"there is a error!"<<std::endl;
}
这是一个简单的循环链表的程序，谁能为我解释一下为什么只有加上红色部分才能正常运行呢？？？


[ 本帖最后由 wghost 于 2010-3-29 16:15 编辑 ]