#include <iostream>
#include <assert.h>
using namespace std;

typedef int DataType;

struct ListNode
{
 ListNode* _next;
 ListNode* _prev;
 DataType _data;

 ListNode(DataType x)
  :_next(NULL)
  , _prev(NULL)
  , _data(x)
 {}
};

typedef ListNode Node;

class List
{

public:
 List()
  :_head(NULL)
  ,_tail(NULL)
 {}

 List(const List& l)
  :_head(NULL)
  ,_tail(NULL)
 {
  Copy(l);
 }

 void Copy(const List& l)
 {
  Node* cur = l._head;
  while (cur)
  {
   PushBack(cur->_data);
   cur = cur->_next;
  }
 }

 List& operator=(const List& l)
 {
  Destory();
  Copy(l);
  return *this;
 }

 ~List()
 {
  Destory();
 }

 void Destory()
 {
  if (_head)
  {
   Node* cur = _head;
   while (_head)
   {
    cur = _head;
    _head = _head->_next;
    delete cur;
   }
   _head = _tail = NULL;
  }
 }

 void PushBack(DataType x)
 {
  if (_head == NULL)
  {
   Node* tmp = new Node(x);
   tmp->_next = tmp->_prev = NULL;
   _head = _tail = tmp;
  }
  else
  {
   Node* tmp = new Node(x);
   _tail->_next = tmp;
   tmp->_prev = _tail;
   _tail = tmp;
  }
 }

 void PopBack()
 {
  if (_head == NULL)
  {
   return;
  }
  else if (_head->_next == NULL)
  {
   delete _head;
   _head = _tail = NULL;
  }
  else
  {
   Node* tmp = _tail;
   _tail = _tail->_prev;
   _tail->_next = NULL;
   delete tmp;
  }
 }

 void PushFront(DataType x)
 {
  if (_head == NULL)
  {
   _head = _tail = new Node(x);
  }
  else
  {
   Node* tmp = new Node(x);
   tmp->_next = _head;
   _head->_prev = tmp;
   _head = _head->_prev;
  }
 }

 void PopFront()
 {
  if (_head == NULL)
  {
   return;
  }
  else if (_head->_next == NULL)
  {
   delete _head;
   _head = _tail = NULL;
  }
  else
  {
   Node* tmp = _head;
   _head = _head->_next;
   delete tmp;
   _head->_prev = NULL;
  }
 }

 Node* Find(DataType x)
 {
  Node* cur = _head;
  while (cur)
  {
   if (cur->_data == x)
    return cur;
   cur = cur->_next;
  }
  return NULL;
 }

 // 在pos的前面插入x
 void Insert(Node* pos, DataType x)
 {
  assert(pos);
  if ((pos == 0) || (pos->_prev == NULL))
  {
   PushFront(x);
  }
  else
  {
   Node* font = pos->_prev;
   Node* tmp = new Node(x);
   tmp->_prev = font;
   tmp->_next = pos;
   font->_next = tmp;
   pos->_prev = tmp;
  }
 }

 //删除pos位置的元素
 void Erase(Node* pos)
 {
  assert(pos);
  if ((pos == 0) || (pos->_prev == NULL))
  {
   PopFront();
  }
  else if (pos->_next == NULL)
  {
   PopBack();
  }
  else
  {
   Node* font = pos->_prev;
   Node* last = pos->_next;
   font->_next = last;
   last->_prev = font;
   delete pos;
  }
 }

 //逆序整个双链表
 void Reverse()
 {
  Node* cur = _head;
  while (cur)
  {
   swap(cur->_next,cur->_prev);
   cur = cur->_prev;
  }
  swap(_head, _tail);
 }


 void Print()
 {
  Node* cur = _head;
  while (cur)
  {
   cout << cur->_data << "->";
   cur = cur->_next;
  }
  cout << "NULL" << endl;
 }

private:
 Node* _head;
 Node* _tail;
};