0230. Kth Smallest Element in a B S T

# 230. Kth Smallest Element in a BST#

## 题目 #

Given a binary search tree, write a function `kthSmallest` to find the kth 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?

## 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

## 代码 #

``````
package leetcode

/**
* Definition for a binary tree node.
* type TreeNode struct {
*     Val int
*     Left *TreeNode
*     Right *TreeNode
* }
*/
func kthSmallest(root *TreeNode, k int) int {
res, count := 0, 0
inorder230(root, k, &count, &res)
return res
}

func inorder230(node *TreeNode, k int, count *int, ans *int) {
if node != nil {
inorder230(node.Left, k, count, ans)
*count++
if *count == k {
*ans = node.Val
return
}
inorder230(node.Right, k, count, ans)
}
}

``````

Apr 8, 2023