算法 2.22
void ListDelete_DuL(DuLinkList &L, DuNode* p, ElemType &e) 
{
      // 刪除帶頭結(jié)點的雙向循環(huán)鏈表L中p結(jié)點，并以e返回它的數(shù)據(jù)元素
      e = p->data;
      p->prior->next = p->next;
      p->next->prior = p->prior;
      delete p;    
} // ListDelete_DuL