模仿STL中list，实现了其大部分功能。list可以高效地利用内存资源，常数时间的插入删除操作。并且，list除了erase外，不怎么存在迭代器失效的现象。

#include<iostream>
#include<iterator>
#include<algorithm>
using namespace std;
template<class T>
struct _List_node{
	typedef void* void_pointer;
	void_pointer next;
	void_pointer prev;
		T data;
};
template<class T,class Ref,class Ptr>
class _List_iterator{
public:
	typedef _List_iterator<T, T&, T*> iterator;
	typedef _List_iterator<T, const T&, const T*> const_iterator;
	typedef _List_iterator<T, Ref, Ptr> self;
	typedef T value_type;
	typedef Ptr pointer;
	typedef Ref reference;
	typedef _List_node<T>* link_type;
	typedef size_t size_type;
	typedef ptrdiff_t difference_type;
	link_type node;
	_List_iterator(link_type x):node(x){}
	_List_iterator(){}
	_List_iterator(const iterator &x):node(x.node){}
	bool operator== (const iterator& x )const{return node == x.node;}
	bool operator!= (const iterator& x)const{return node != x.node;}
	reference operator*()const{return (*node).data;}
	pointer operator->()const{return &(operator*());}
	T operator *(){return node->data;}
	iterator &operator++(int){
		iterator tmp = *this;
		++*this;
		return tmp;
	}
	iterator& operator++(){
		node = (link_type)((*node).next);
		return *this;
	}
	iterator& operator--(){
		node = (link_type)((*node).prev);
		return *this;
	}
	iterator& operator--(int){
		iterator tmp = *this;
		--*this;
		return tmp;
	}
};
template<class T>
class List{
public:
	typedef _List_node<T>* link_type;
	typedef _List_iterator<T,T&,T*> iterator;
	typedef T value_type;
	typedef T* pointer;
	typedef T& reference;
	typedef size_t size_type;
	typedef ptrdiff_t difference_type;
protected:
	typedef _List_node<T> list_node;
	link_type node;
	//.....
public:
	List(){create_node();}
	List(List& alist){
		create_node();
		for(List<T>::iterator ite=alist.begin();ite!=alist.end();++ite)
			push_back(*ite);
	}
	iterator begin(){return (link_type)((*node).next);}
	iterator end(){return node;}
	bool empty()const{return node->next == node;}
	size_type size()const{
		size_type result = 0;
		distance(begin(),end(),result);
		return result;
	}
	reference front(){return *begin();}
	reference back(){return *(--end());}
	iterator insert(iterator position,const T& x){
		link_type tmp = create_node(x);
		tmp->next = position.node;
		tmp->prev = position.node->prev;
		(link_type(position.node->prev))->next = tmp;
		position.node->prev = tmp;
		return tmp;
	}
	link_type create_node(const T& x){
		link_type p = new list_node;
		p->data = http://www.mamicode.com/x;>