C++实现双向链表

#pragma once 
#include <stdio.h>
#include <iostream>
#include <assert.h>
using namespace std;
typedef int DataType;
struct ListNode
{
  ListNode* _next;   //存放下一个节点地址
  ListNode* _prev;     //存放上一个节点地址
  DataType _data;
  ListNode(DataType x)
    :_data(x)
    , _next(NULL)
    , _prev(NULL)
  {}
};
class List
{
  typedef ListNode Node;
public:
  List()
    :_head(new Node(DataType()))
  {
    _head->_next = _head;
    _head->_prev = _head;
  }
  List(const List& l)
    :_head(new Node(DataType()))
  {
    _head->_next = _head;
    _head->_prev = _head;
    Node* cur = l._head->_next;
    while (cur != l._head)
    {
      PushBack(cur->_data);
      cur = cur->_next;
    }
  }
  List& operator=(List& l)
  {
    if (this != &l)
    {
      swap(_head, l._head);
    }
    return *this;
  }
  ~List()
  {
    Node* cur = _head->_next;
    while (cur != _head)
    {
      Node* next = cur->_next;
      delete cur;
      cur = next;
    }
    delete _head;
    _head = NULL;
  }
  void Print() const
  {
    Node* cur = _head->_next;
    cout << "head->";
    while (cur != _head)
    {
      cout << cur->_data << "->";
      cur = cur->_next;
    }
    cout << endl;
    Node* tail = _head->_prev;
    while (tail != _head)
    {
      cout << tail->_data << "->";
      tail = tail->_prev;
    }
    cout << "head" << endl;
  }
  void PushBack(DataType x);
  void PushFront(DataType x);
  void PopBack();
  void PopFront();
  ListNode* Find(DataType x);
  void Insert(Node* pos, DataType x);
  void Erase(Node* pos);
private:
  Node* _head;
};
void List::PushBack(DataType x)
{
  Node* tail = _head->_prev;
  Node* new_node = new Node(x);
  tail->_next = new_node;
  new_node->_prev = tail;
  new_node->_next = _head;
  _head->_prev = new_node;
  //Insert(_head, x);
}
void List::PushFront(DataType x)
{
  Node* cur = _head->_next;
  Node* new_node = new Node(x);
  new_node->_next = cur;
  cur->_prev = new_node;
  new_node->_prev = _head;
  _head->_next = new_node;
  //Insert(_head->_next, x);
}
void List::PopBack()
{
  Node* to_delete = _head->_prev;
  Node* cur = to_delete->_prev;
  cur->_next = _head;
  _head->_prev = cur; 
  delete to_delete;
  //Erase(_head->_prev);
}
void List::PopFront()
{
  Node* to_delete = _head->_next;
  Node* cur = to_delete->_next;
  cur->_prev = _head;
  _head->_next = cur;
  delete to_delete;
  //Erase(_head->_next);
}
ListNode* List::Find(DataType x)
{
  Node* cur = _head->_next;
  while (cur != _head)
  {
    if (cur->_data == x)
    {
      return cur;
    }
    cur = cur->_next;
  }
  return NULL;
}
void List::Insert(Node* pos, DataType x)
{
  assert(pos);
  Node* prev = pos->_prev;
  Node* new_node = new Node(x);
  new_node->_next = pos;
  pos->_prev = new_node;
  prev->_next = new_node;
  new_node->_prev = prev;
}
void List::Erase(Node* pos)
{
  assert(pos);
  Node* prev = pos->_prev;
  Node* next = pos->_next;
  prev->_next = next;
  next->_prev = prev;
  delete pos;
}
void TestList()
{
  List l;
  l.PushBack(1);
  l.PushBack(2);
  l.PushBack(3);
  l.PushBack(4);
  l.PopBack();
  l.Print();
  ListNode* pos = l.Find(2);
  printf("pos->_data expext 2, actual %d:[%p]\n", pos->_data, pos);
  pos = l.Find(4);
  printf("pos->_data expext NULL, actual [%p]\n", pos);
  pos = l.Find(1);
  printf("pos->_data expext 1, actual %d:[%p]\n", pos->_data, pos);
  l.Insert(pos, 0);
  l.Print();
  l.Erase(pos);
  l.Print();
  List l1(l);
  l1.PushFront(8);
  l1.PushFront(7);
  l1.PushFront(6);
  l1.PushFront(5);
  l1.PopFront();
  l1.Print();
  List l2;
  l2 = l;
  l2.Print();
}

#include "List.h"
int main()
{
  cout << "双向链表：" << endl;
  TestList();
  return 0;
}