Problem Description

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

Examples

Example 1:

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

Example 2:

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

Constraints:

• The number of nodes in the tree is n.
• 1 <= k <= n <= 104
• 0 <= Node.val <= 104

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

解析

給定一個二元搜尋樹的根結點 root 還有一個數字正整數 k

要求寫一個演算法找出排序由小至大第 k 個元素

在二元搜尋要找出第 k 小的元素

需要知道二元搜尋樹特性