C++实现循环双向链表

typedef int DataType;
struct Node
{
    struct Node* _pNext;
    struct Node* _pPre;
    DataType _data;

    Node(const DataType& data = DataType())
        : _pNext(NULL)
        , _pPre(NULL)
        , _data(data)
    {}
};


class List
{
    typedef Node* PNode;
public:
    List()
        : _pHead(NULL)
    {}

    List(const DataType* array, size_t size)
    {
        for (size_t i = ; i < size; ++i)
        {
            PushBack(array[i]);
        }
    }
    List(const List& L)
    {
        _pHead = new Node;
        PNode ptail = _pHead->_pNext;
        while (ptail != _pHead)
        {
            PushBack(ptail->_data);
            ptail = ptail->_pNext;
        }
    }
    List& operator=(const List& s)
    {
        if (this == &s)
        {
            return *this;
        }
        this->_pHead = new Node;
        PNode ptail = _pHead->_pNext;
        while (ptail != _pHead)
        {
            PushBack(ptail->_data);
            ptail = ptail->_pNext;
        }
    }

    ~List() //逆向销毁
    {
        Clear();//清空链表中的元素，表头结点还在
        delete _pHead;
        _pHead = NULL;
    }

    void PushBack(const DataType& data)
    {
        PNode ptail = _pHead->_pPre;
        PNode pNewNode = new Node(data);
        ptail->_pNext = pNewNode;
        pNewNode->_pNext = _pHead;
        pNewNode->_pPre = ptail;
        _pHead->_pPre = pNewNode;
    }
    void PopBack()
    {
        PNode ptail = _pHead->_pPre;
        ptail->_pPre->_pNext = _pHead;
        _pHead->_pPre = ptail->_pPre;
        delete ptail;
    }
    void PushFront(const DataType& data)
    {
        PNode ptail = new Node(data);
        ptail->_pNext = _pHead->_pNext;
        _pHead->_pNext->_pPre = ptail;
        ptail->_pPre = _pHead;
        _pHead->_pNext = ptail;
    }
    void PopFront()
    {
        if (_pHead->_pNext == _pHead)
            return;
        else
        {
            PNode ptail = _pHead->_pNext;
            _pHead->_pNext = ptail->_pNext;
            ptail->_pNext->_pPre = _pHead;
            delete ptail;
        }
    }

    // 返回插入的新节点 
    Node* Insert(Node* pos, DataType data)
    {
        PNode ptail = pos->_pPre;
        PNode pNewNode = new Node(data);
        pNewNode->_pNext = ptail;
        ptail->_pPre = pNewNode;
        pos->_pNext = pNewNode;
        pNewNode->_pPre = pos;
    }
    // 返回删除节点的下一个位置 
    Node* Erase(Node* pos)
    {
        pos->_pPre->_pNext = pos->_pNext;
        pos->_pNext->_pPre = pos->_pPre;
        delete pos;
    }
    size_t Size()const
    {
        int count = ;
        PNode ptail = _pHead->_pNext;
        while (ptail != _pHead)
        {
            count++;
            ptail = ptail->_pNext;
        }
        return count;
    }
    bool Empty()const
    {
        return _pHead->_pNext - _pHead;
    }
    void Clear()
    {
        PNode ptail = _pHead->_pNext;
        while (ptail != _pHead)
        {
            _pHead->_pNext = ptail->_pNext;
            delete ptail;
            ptail = _pHead->_pNext;
        }
        ptail->_pPre = _pHead;
    }

    friend ostream& operator<<(ostream& _cout, const List& s)
    {
        PNode ptail = _pHead->_pNext;
        while (ptail != _pHead)
        {
            cout << s._pHead->_data << ;
            ptail = ptail->_pNext;
        }
        return _cout;
    }
private:
    Node* _pHead;
};

int main(void)
{
    size_t i;
    List l1;
    l1.PushBack();
    l1.PushBack();
    l1.PushBack();
    l1.PushBack();
    l1.PopBack();

    cout << l1 << endl;
    return ;
}