230. Kth Smallest Element in a BST

Published on August 6, 2023

Question

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.

Constraints:

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

Algorithm

This question is much similar to the question of BST iterator but a little bit easier. We only need to record the index of current number and pop the right one when the index == k, no matter you are using recursive or iterative way.

Code

/**
 * 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 res;
    List<Integer> list = new ArrayList<>();
    public int kthSmallest(TreeNode root, int k) {
        helper(root, k);
        return res;
    }
    
    private void helper(TreeNode root, int k) {
        if (root == null) return;
        helper(root.left, k);
        list.add(root.val);
        if (list.size() == k) {
            res = list.get(list.size() - 1);
            return ;
        }
        helper(root.right, k);
    }
}

There is no need to store the list, a count is enough.

class Solution {
    int res;
    int count;
    public int kthSmallest(TreeNode root, int k) {
        res = 0;
        count = k;
        helper(root);
        return res;
    }
    
    private void helper(TreeNode root) {
        if (root == null) return;
        helper(root.left);
        count--;
        if (count == 0) {
            res = root.val;
        }
        helper(root.right);
    }
}