list库实现的要点：

构建list类时，需要同时构建struct Node来存储节点信息，list类中只存储哨兵位节点信息，迭代器类需要template<T,Ptr,Ref>来构建const和非const迭代器，迭代器中也是存储节点信息。反向迭代器也是同样道理，但是可以用迭代器来构建反向迭代器。具体代码如下 

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

template<class T>
struct __list_node
{
	__list_node(const T& val = T())
		:_data(val)
		,_prev(nullptr)
		,_next(nullptr)
		{}

	__list_node* _prev;
	__list_node* _next;
	T _data;
};//构建每个节点

//构建迭代器struct
template<class T, class Ptr, class Ref>
struct __list_iterator
{
	typedef __list_node<T> Node;
	typedef __list_iterator<T, Ptr, Ref> Self;
	Node* _node;//成员变量还是节点，只是在成员函数做手脚
	__list_iterator(Node* val )
		:_node(val)
	{}
	
	Self& operator++()
	{
		_node = _node->_next;
		return *this;
	}
	Self operator++(int)//后置
	{
		Self tmp = *this;
		++(*this);
		return tmp;
	}
	Self& operator--()
	{
		_node = _node->_prev;
		return *this;
	}
	Self operator--(int)
	{
		Self tmp = *this;
		--(*this);
		return tmp;
	}
	
	Ptr operator->()
	{
		return &(_node->_data);
	}
	Ref operator*()
	{
		return _node->_data;
	}
	bool operator!= ( const Self& val )
	{
		return !(_node == val._node);
	}


};
template<class T,class Ptr,class Ref>
struct __list_reverse_iterator
{
	typedef __list_iterator<T, T*, T&> iterator;
	typedef __list_reverse_iterator<T, T*, T&> Self;
	iterator _it;
	__list_reverse_iterator(iterator it)
		:_it(it)
	{}
	Self operator++()
	{
		_it = _it._node->_prev;
		return *this;
	}
	T& operator*()
	{
		return _it._node->_data;
	}
	bool operator!=(const Self& rit)
	{
		return !(_it._node == rit._it._node);
	}
	
};

//构建双向带头循环链表
template<class T>
class list
{
	typedef __list_node<T> Node;
public:
	typedef __list_iterator<T,T*,T&> iterator;
	typedef __list_iterator<T, const T*, const T&> const_iterator;
	typedef __list_reverse_iterator<T, T*, T&> reverse_iterator;
	reverse_iterator rbegin()
	{
		return reverse_iterator(--end());
	}
	reverse_iterator rend()
	{
		return reverse_iterator(end());
	}
	iterator begin()
	{
		return iterator(_phead->_next);
	}
	const_iterator begin()const
	{
		return const_iterator(_phead->_next);
	}
	iterator end()
	{
		return iterator(_phead);
	}
	const_iterator end()const
	{
		return const_iterator(_phead);
	}

	list()//开头空间，初始化
	{
		_phead = new Node;
		_phead->_next = _phead;
		_phead->_prev = _phead;
	}
	~list()
	{
		clear();
		delete _phead;
		_phead = nullptr;
	}
	//拷贝构造(先创建一个哨兵位，然后再pushback)
	list(const list<T>& l)
	{
		_phead = new Node;
		_phead->_next = _phead;
		_phead->_prev = _phead;
		for (auto& x : l)
		{
			push_back(x);
		}
	}
	list<T>& operator=(const list<T>& l)
	{
		list<T> tmp(l);
		swap(_phead, tmp._phead);
		return *this;
	}

	//尾插入
	void push_back(const T& val)
	{
		//Node* tail = _phead->_prev;
		//Node* newnode = new Node(val);
		更变链接
		//tail->_next = newnode;
		//newnode->_prev = tail;
		//newnode->_next = _phead;
		//_phead->_prev = newnode;
		insert(end(), val);
	}
	//头插
	void push_front(const T& val)
	{
		insert(begin(), val);
	}
	//头删除
	void pop_front()
	{
		erase(begin());
	}
	//尾删除
	void pop_back()
	{
		erase(--end());
	}
	//清空节点(除了哨兵位，都清除)
	void clear()
	{
		iterator it = begin();
		while (it != end())
		{
			it = erase(it);
		}
	}
	//随机插入
	void insert(iterator pos, const T& val)
	{
		Node* pcur = pos._node;
		Node* prev = pcur->_prev;
		Node* newnode = new Node(val);
		//构建联系
		newnode->_prev = prev;
		newnode->_next = pcur;
		prev->_next = newnode;
		pcur->_prev = newnode;
		
	}
	//删除指定位置(不能将哨兵位删掉！！
	iterator erase(iterator pos)
	{
		assert(pos != end());
		Node* pcur = pos._node;
		Node* prev = pcur->_prev;
		Node* next = pcur->_next;
		delete pcur;
		pcur = nullptr;
		prev->_next = next;
		next->_prev = prev;
		return iterator(next);
	}

private:
	Node* _phead;
};