[leetcode #230] Kth Smallest Element in a BST

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Problem

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.
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?

Idea

これは,与えられたBSTにおいてk番目に小数を求める問題である.
BSTをシーケンスとして探索し,ノード数を計算した.ノード数がkの場合、対応する値を返します.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int cnt = 0;
    int ans = 0;

    public int kthSmallest(TreeNode root, int k) {
        inorder(root, k);

        return ans;
    }

    private void inorder(TreeNode node, int k) {
        if (node == null) {
            return;
        }

        inorder(node.left, k);
        cnt++;
        if (cnt == k) {
            ans = node.val;
        }
        inorder(node.right, k);
    }
}

Reference

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

Reference

この問題について([leetcode #230] Kth Smallest Element in a BST), 我々は、より多くの情報をここで見つけました https://velog.io/@timevoyage/leetcode-230-Kth-Smallest-Element-in-a-BST

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