Kth Smallest Element in a BST

Kth Smallest Element in a BST

Description

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

Tags: Binary Search

解读题意

找出二叉搜索树中第k个大的元素

思路1

这是一道典型的二叉搜索树遍历问题。对二叉搜索树进行中序遍历可以得到有序的数组，访问其对应元素即可。

public class Solution {

    private int count = 0;
    private int kth = 0;

    public int kthSmallest(TreeNode root, int k) {

        if (root == null || k <= 0)
            return 0;

        // BST中序遍历得到有序列表
        midOrder(root,k);

        return kth;
    }

    private void midOrder(TreeNode root, int k) {

        if (root == null)
            return;

        midOrder(root.left, k);
        count++;
        if (k == count)
            kth = root.val;
        midOrder(root.right, k);
    }
}

• time complexity:O(n)。

leetCode汇总：https://blog.csdn.net/qingtian_1993/article/details/80588941

项目源码，欢迎star：https://github.com/mcrwayfun/java-leet-code