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230. Kth Smallest Element in a BST

Medium

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?

Solution

using LeetCodeNet.Com_github_leetcode;

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

    public int KthSmallest(TreeNode root, int k) {
        this.k = k;
        Calculate(root);
        return val;
    }

    private void Calculate(TreeNode node) {
        if (node.left == null && node.right == null) {
            count++;
            if (count == k) {
                this.val = (int) node.val;
            }
            return;
        }
        if (node.left != null) {
            Calculate(node.left);
        }
        count++;
        if (count == k) {
            this.val = (int) node.val;
            return;
        }
        if (node.right != null) {
            Calculate(node.right);
        }
    }
}

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