# [230. Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)


## 题目

Given a binary search tree, write a function `kthSmallest` to find the **k**th smallest element in it.

**Note:** You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

**Example 1:**

    Input: root = [3,1,4,null,2], k = 1
       3
      / \
     1   4
      \
       2
    Output: 1

**Example 2:**

    Input: root = [5,3,6,2,4,null,null,1], k = 3
           5
          / \
         3   6
        / \
       2   4
      /
     1
    Output: 3

**Follow up:**What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?


## 题目大意

给定一个二叉搜索树，编写一个函数 kthSmallest 来查找其中第 k 个最小的元素。你可以假设 k 总是有效的，1 ≤ k ≤ 二叉搜索树元素个数。


## 解题思路

- 由于二叉搜索树有序的特性，所以中根遍历它，遍历到第 K 个数的时候就是结果