循环链表以及迭代器的使用c++

循环链表，顺便也附上链表的迭代器

#include <iostream>

using namespace std;

template<class T> class List;
template<class T> class ListIterator;

template<class T>
class ListNode
{
    friend class List<T>;
    friend class ListIterator<T>;
private:
    T data;
    ListNode *Link;
    ListNode(T);
    ListNode(){}
};

template<class T>
ListNode<T>::ListNode(T element)
{
    data = element;
    Link = 0;
}

template<class T>
class List
{
    friend class ListIterator<T>;
public:
    List(){first=new ListNode<T>; first->Link=first;}
    void Insert(T);
    void Delete(T);
private:
    ListNode<T> *first;
};

template<class T>
class ListIterator
{
public:
    ListIterator(const List<T>& l):mylist(l),current(l.first->Link){}
    bool NotNull();
    bool NextNotNull();
    T* First();
    T* Next();
private:
    const List<T> &mylist;
    ListNode<T>* current;
};

template<class T>
bool ListIterator<T>::NotNull()
{
    if(current != mylist.first) return true;
    else return false;
}

template<class T>
bool ListIterator<T>::NextNotNull()
{
    if(current->Link != mylist.first) return true;
    else return false;
}

template<class T>
T* ListIterator<T>::First()
{
    if(NotNull()) return &current->data;
    else return 0;
}

template<class T>
T* ListIterator<T>::Next()
{
    current = current->Link;
//    if(current==mylist.first)current=current->Link;
    return &current->data;
}

template<class T>
void List<T>::Insert(T x)
{
    ListNode<T> *newNode=new ListNode<T>(x);
    newNode->Link = first->Link;
    first->Link = newNode;
}

template<class T>
void List<T>::Delete(T k)
{
    ListNode<T> *previous = first;
    ListNode<T> *current;
    for(current=first->Link; (current!=first)&&(current->data!=k);
        previous=current, current=current->Link);
    if(current!=first)
    {
        previous->Link = current->Link;
        delete current;
    }
}

int main()
{
    cout << "Hello world!" << endl;

    List<int> intlist;
    intlist.Insert(5);
    intlist.Insert(15);
    intlist.Insert(25);
    intlist.Insert(35);

    ListIterator<int> myl(intlist);
    if(myl.NotNull())
    {
        cout << *myl.First()*10;
        while(myl.NextNotNull())
              cout << "->" << *myl.Next()*10;
        cout << endl;
    }

    return 0;
}