leetcode-230-Kth-Smallest-Element-in-a-BST

发表于   |  

描述

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:

1
2
3
4
5
6
7
Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

1
2
3
4
5
6
7
8
9
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?

分析

借助两个全局变量，一个存放结果，一个存放当前还需要遍历的节点。中序遍历，直到节点遍历完 k 个，返回结果。

解决方案1(Java)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int result = 0;
    private int count = 0;
    public int kthSmallest(TreeNode root, int k) {
        count = k;
        dfs(root);
        return result;
    }
    private void dfs(TreeNode root) {
        if (root.left != null) {
            dfs(root.left);
        }
        count--;
        if (count == 0) {
            result = root.val;
        }
        if (root.right != null) {
            dfs(root.right);
        }
    }
}

相关问题

题目来源