230. Kth Smallest Element in a BST

难度: Medium

题目描述

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?

解题思路

代码实现

解决方案

java

/**
 * 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 kTh = 0;
    int kVal = 0;

    public int kthSmallest(TreeNode root, int k) {
        inorderTraversal(root, k);
        return kVal;

    }

    public void inorderTraversal(TreeNode root, int k) {
        if (root == null) {
            return;
        }

        inorderTraversal(root.left, k);
        kTh++;
        if (k == kTh) {
            kVal = root.val;
            return;
        }
        inorderTraversal(root.right, k);
    }
}