双链表的实现

双链表：可以从一个表结点出发，在线性表中随意访问它的前驱结点和后继结点，双链表有两个指针。

双链表结template<class Elem> class Link   private:       static Link<Elem>* freelist ;//Head of the freelis   public:

       Elem element;//value for this node
       Link *next;//pointer to next  node in list
       Link *prev;//pointer to previous node
       Link(const Elem& e, Link *prevp=NULL,Link *nextp=NULL)
            {   element =e;
                 prev=prevp;
                 next=nextp;
              }    
      Link(Link* prevp=NULL,Link* nextp=NULL)
            {
                 prev=prevp;
                next=nextp;
             }
      //Overload new and delete operators for freelist
     void* operator new(size_t);
     void opterator delete(void*);

};

template<class Elem>
Link<Elem>*Link<Elem>::freelist=NULL;

template<class Elem>//Overload for new operator
void* Link<Elem>::operator new(size_t){
    if(freelist==NULL) return :: new Link;//Create space
    Link<Elem>* temp=freelist;//can take from freelist
    freelist=freelist->next;
    return temp;
}

template<class Elem>//Overload for delete operator
void Link<Elem>::Operator delete(void *ptr){
    ((Link<Elem>*)ptr)->next=freelist;//put on freelist
    freelist=(Link<Elem>*)ptr;
}

　　双链表类insert,append,remove和prev函数的实现

tempate<class Elem>//insert at front of right partition
bool LLink<Elem>::insert(const Elem& item){
   fence->next=new Link<Elem>(item,fence,fence->next) ;
   if(fence->next->next!=NULL)//if not deleting at end
      fence->next->next->prev=fence->next;
   if(tail==fence)
      tail=fence->next;
   rightcnt++;
   return true; 
}

template<class Elem>//append Elem to end of the list
bool LList<Elem>::append(const Elem& item){
   tail=tail->next=new Link<Elm>(item,tail,NULL);
   rightcnt++;
   return true;
}

template<class Elem>//remove and return first Elem in right partion
bool LList<Elem>::remove(Elem& it){
   if(fence->next==NULL) return false;//empty right
   it=fence->next->element;//remember value
   Link<Elem>* ltemp=fence->next;//remenber link node
   if(ltemp->next!=NULL) ltemp->next-prev=fence;
   else tail=fence;//reset tail
   fence->next=ltemp->next;
   delete ltemp;
   rightcnt--;
   return true;
}

template<class Elem>//move fence one step left;no change if left is empty
void LList<Elem>::prev(){
   if(fence!=head)
   {
       fence=fence->prev;
       leftcnt--;
       rightcent++;
    }
}