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?

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

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)
	}
}

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