AVL_树
将AVL树实现为具有双向迭代器Class接口的容器:
template < class xss=removed>>
class BST {
public:
BST ();
BST ( const BST &);
BST& operator = ( const BST &);
template < class>
void assign (InputIt, InputIt);
iterator begin ();
const_iterator cbegin ();
iterator end ();
const_iterator cend ();
iterator insert ( const T &);
iterator remove ( const T &);
i