# [173. Binary Search Tree Iterator](https://leetcode.com/problems/binary-search-tree-iterator/)


## 题目

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling `next()` will return the next smallest number in the BST.

**Example:**

![](https://assets.leetcode.com/uploads/2018/12/25/bst-tree.png)

    BSTIterator iterator = new BSTIterator(root);
    iterator.next();    // return 3
    iterator.next();    // return 7
    iterator.hasNext(); // return true
    iterator.next();    // return 9
    iterator.hasNext(); // return true
    iterator.next();    // return 15
    iterator.hasNext(); // return true
    iterator.next();    // return 20
    iterator.hasNext(); // return false

**Note:**

- `next()` and `hasNext()` should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
- You may assume that `next()` call will always be valid, that is, there will be at least a next smallest number in the BST when `next()` is called.


## 题目大意

实现一个二叉搜索树迭代器。你将使用二叉搜索树的根节点初始化迭代器。调用 next() 将返回二叉搜索树中的下一个最小的数。

## 解题思路

- 用优先队列解决即可