230. Kth Smallest Element in a BST

Bertha 。 2022-03-08 12:50 144阅读 0赞

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?

难度：medium

题目：给定二叉搜索树，找出其值第K小的结点。

思路：中序遍历

Runtime: 0 ms, faster than 100.00% of Java online submissions for Kth Smallest Element in a BST.  
Memory Usage: 38.9 MB, less than 19.71% of Java online submissions for Kth Smallest Element in a BST.

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        public int kthSmallest(TreeNode root, int k) {
            int[] result = {root.val};
            kthSmallest(root, k, new int[1], result);
            
            return result[0];
        }
        
        public void kthSmallest(TreeNode root, int k, int[] count, int[] result) {
            if (root == null || count[0] >= k) {
                return;
            }
            kthSmallest(root.left, k, count, result);
            count[0]++;
            if (count[0] == k) {
                result[0] = root.val;
                return;
            } 
            kthSmallest(root.right, k, count, result);
        }
    }