diff --git a/TinySTL/AVLTree.h b/TinySTL/AVLTree.h
index 3912e6e..fa13409 100644
--- a/TinySTL/AVLTree.h
+++ b/TinySTL/AVLTree.h
@@ -2,13 +2,17 @@
 #define _AVL_TREE_H_
 
 #include "Allocator.h"
+#include "Stack.h"
+
+#include <set>
 
 namespace TinySTL{
 	//class of avl tree
 	template<class T>
 	class avl_tree{
-	public:
-		typename T value_type;
+	private:
+		template<class T>
+		friend class avl_iter;
 	private:
 		struct node{
 			T data_;
@@ -18,13 +22,23 @@ namespace TinySTL{
 				:data_(d), left_(l), right_(r), height_(h){}
 		};
 		typedef TinySTL::allocator<T> dataAllocator;
+	public:
+		typedef T value_type;
+		typedef avl_iter<node> const_iterator;
+		typedef const T& const_reference;
 	private:
 		node *root_;
+		size_t size_;
 	public:
-		avl_tree() :root_(0){}
+		avl_tree() :root_(0), size_(0){}
 		avl_tree(const avl_tree&) = delete;
 		avl_tree& operator = (const avl_tree&) = delete;
 		~avl_tree(){ destroyAndDeallocateAllNodes(root_); }
+
+		size_t height()const{ return root_ == 0 ? -1 : root_->height_; }
+		size_t size()const{ return size_; }
+		bool empty()const{ return root_ == 0; }
+		/*const_iterator root(){ return const_iterator(root_, this); }*/
 	private:
 		void destroyAndDeallocateAllNodes(node *p){
 			if (p != 0){
@@ -35,6 +49,116 @@ namespace TinySTL{
 			}
 		}
 	};
+
+	namespace{
+		//class of bst iterator
+		template<class T>//T = node
+		class avl_iter{
+		private:
+			template<class T>
+			friend class avl_tree;
+		private:
+			typedef typename avl_tree<typename T::value_type>::vaule_type value_type;
+			typedef typename avl_tree<typename T::value_type>::const_reference const_reference;
+			typedef typename const T::value_type *const_pointer;
+			typedef avl_tree<typename T::value_type> * cntrPtr;
+		private:
+			const T *ptr_;
+			cntrPtr container_;
+			stack<const T *> parent_;//保存从root到ptr_的父节点的路径
+			std::set<const T *> visited_;
+		public:
+			avl_iter(const T *ptr, cntrPtr container)
+				:ptr_(ptr), container_(container){
+				auto temp = container_->root_;
+				while (temp &&ptr_ && temp != ptr_ && temp->data_ != ptr_->data_){
+					parent_.push(temp);
+					if (temp->data_ < ptr_->data_)
+						temp = temp->right_;
+					else if (temp->data_ > ptr_->data_)
+						temp = temp->left_;
+				}
+			}
+
+			operator const T*(){ return ptr_; }
+			const_reference operator*(){ return ptr_->data_; }
+			const_pointer operator ->(){ return &(operator*()); }
+
+			avl_iter left()const{ return avl_iter(ptr_->left_, container_); }
+			avl_iter right()const{ return avl_iter(ptr_->right_, container_); }
+
+			avl_iter& operator ++();
+			avl_iter operator ++(int);
+			avl_iter& operator --();
+			avl_iter operator --(int);
+		public:
+			template<class T>
+			friend bool operator ==(const avl_iter<T>& it1, const avl_iter<T>& it2);
+			template<class T>
+			friend bool operator !=(const avl_iter<T>& it1, const avl_iter<T>& it2);
+		};//end of avl_iter
+		template<class T>
+		avl_iter<T>& avl_iter<T>::operator ++(){
+			visited_.insert(ptr_);//设为已访问
+			if (ptr_->right_ != 0){
+				parent_.push(ptr_);
+				ptr_ = ptr_->right_;
+				while (ptr_ != 0 && ptr_->left_ != 0){
+					parent_.push(ptr_);
+					ptr_ = ptr_->left_;
+				}
+			}
+			else{
+				if (!parent_.empty() && visited_.count(parent_.top()) == 0){//父节点未访问
+					ptr_ = parent_.top();
+					parent_.pop();
+				}
+				else{
+					ptr_ = 0;
+				}
+			}
+			return *this;
+		}
+		template<class T>
+		avl_iter<T> avl_iter<T>::operator ++(int){
+			auto res = *this;
+			++*this;
+			return res;
+		}
+		template<class T>
+		avl_iter<T>& avl_iter<T>::operator --(){
+			visited_.erase(ptr_);//设为未访问
+			if (ptr_->left_ != 0){
+				parent_.push(ptr_);
+				ptr_ = ptr_->left_;
+				while (ptr_ && ptr_->right_){
+					parent_.push(ptr_);
+					ptr_ = ptr_->right_;
+				}
+			}
+			else{
+				if (!parent_.empty() && visited_.count(parent_.top()) == 1){//父节点已经访问了
+					ptr_ = parent_.top();
+					parent_.pop();
+				}
+			}
+			return *this;
+		}
+		template<class T>
+		avl_iter<T> avl_iter<T>::operator --(int){
+			auto res = *this;
+			--*this;
+			return res;
+		}
+		template<class T>
+		bool operator ==(const avl_iter<T>& it1, const avl_iter<T>& it2){
+			return it1.ptr_ == it2.ptr_;
+		}
+		template<class T>
+		bool operator !=(const avl_iter<T>& it1, const avl_iter<T>& it2){
+			return !(it1 == it2);
+		}
+	}
 }
 
 #endif
\ No newline at end of file