p230

发表于 | 分类于 |

LeetCode 230 Kth Smallest Element in a BST 题解

1.题目：

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.
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?

题意：

输入一个二叉查找树和k,寻找这个数中第K小的数。

2.解题思路:

画张图跟如下代码走一遍应该就明白了。

3.代码

[title] [] [url] [link text]
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
31
32
33
34
35
36
37
38
 

/**
 * 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 ans = 0;
		TreeNode knode = root;

		int c = count(knode.left);
		if (c + 1 == k)
			return knode.val;

		if (c < k)
			return kthSmallest(knode.right, k - 1 - c);// 往右的差值
		if (c + 1 > k)
			return kthSmallest(knode.left, k);

		return ans;
	}

	public int count(TreeNode root) {

		if (root == null)
			return 0;

		return 1 + count(root.left) + count(root.right);
	}
}

4.一些总结：