my_list

template<class T>
struct ListNode
{
	ListNode<T>* _next;
	ListNode<T>* _prev;
	T _data;

	ListNode(const T& x = T())
		:_next(nullptr)
		, _prev(nullptr)
		, _data(x)
	{}
};

// typedef ListIterator<T, T&, T*> iterator;
// typedef ListIterator<T, const T&, const T*> const_iterator;

template<class T, class Ref, class Ptr>
struct ListIterator
{
	typedef ListNode<T> Node;
	typedef ListIterator<T, Ref, Ptr> Self;

	Node* _node;

	ListIterator(Node* node)
		:_node(node)
	{}

	Ref operator*()
	{
		return _node->_data;
	}

	Ptr operator->()
	{
		return &_node->_data;
	}

	// ++it
	Self& operator++()
	{
		_node = _node->_next;
		return *this;
	}

	Self operator++(int)
	{
		Self tmp(*this);
		_node = _node->_next;

		return tmp;
	}

	Self& operator--()
	{
		_node = _node->_prev;
		return *this;
	}

	Self operator--(int)
	{
		Self tmp(*this);
		_node = _node->_prev;

		return tmp;
	}

	bool operator!=(const Self& it)
	{
		return _node != it._node;
	}

	bool operator==(const Self& it)
	{
		return _node == it._node;
	}
};

template<class T>
class list
{
	typedef ListNode<T> Node;
public:

	typedef ListIterator<T, T&, T*> iterator;
	typedef ListIterator<T, const T&, const T*> const_iterator;


	iterator begin()
	{
		return _head->_next;
	}

	iterator end()
	{
		return _head;
	}

	const_iterator begin() const
	{
		return _head->_next;
	}

	const_iterator end() const
	{
		return _head;
	}

	void empty_init()
	{
		_head = new Node;
		_head->_next = _head;
		_head->_prev = _head;

		_size = 0;
	}

	list()
	{
		empty_init();
	}

	// lt2(lt1)
	list(const list<T>& lt)
	{
		empty_init();
		for (auto& e : lt)
		{
			push_back(e);
		}
	}

	void swap(list<T>& lt)
	{
		std::swap(_head, lt._head);
		std::swap(_size, lt._size);
	}

	list<T>& operator=(list<T> lt)
	{
		swap(lt);
		return *this;
	}

	void clear()
	{
		iterator it = begin();
		while (it != end())
		{
			it = erase(it);
		}
	}

	~list()
	{
		clear();
		delete _head;
		_head = nullptr;
	}

	/*void push_back(const T& x)
	{
		Node* newnode = new Node(x);
		Node* tail = _head->_prev;

		tail->_next = newnode;
		newnode->_prev = tail;
		newnode->_next = _head;
		_head->_prev = newnode;
	}*/

	void push_back(const T& x)
	{
		insert(end(), x);
	}

	void push_front(const T& x)
	{
		insert(begin(), x);
	}

	void pop_back()
	{
		erase(--end());
	}

	void pop_front()
	{
		erase(begin());
	}

	void insert(iterator pos, const T& val)
	{
		Node* cur = pos._node;
		Node* newnode = new Node(val);
		Node* prev = cur->_prev;

		// prev newnode cur;
		prev->_next = newnode;
		newnode->_prev = prev;
		newnode->_next = cur;
		cur->_prev = newnode;
		_size++;
	}

	iterator erase(iterator pos)
	{
		Node* cur = pos._node;
		Node* prev = cur->_prev;
		Node* next = cur->_next;

		prev->_next = next;
		next->_prev = prev;
		delete cur;
		_size--;

		return iterator(next);
	}

	size_t size() const
	{
		return _size;
	}

	bool empty()
	{
		return _size == 0;
	}

private:
	Node* _head;
	size_t _size;
};