LeetCode 230. Kth Smallest Element in a BST 解题报告

230. Kth Smallest Element in a BST

My Submissions

Question

Total Accepted: 35869  Total Submissions: 100057  Difficulty: Medium

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?

Hint:

1. Try to utilize the property of a BST.
2. What if you could modify the BST node's structure?
3. The optimal runtime complexity is O(height of BST).

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

Subscribe to see which companies asked this question

Show Tags

Show Similar Problems

Have you met this question in a real interview? 

Yes

 

No

Discuss

    这道题要求找出一棵二叉搜索树的第K小的元素。关键在于利用BST的性质。BST的中根遍历序是一个有序的集合。因此中根遍历的第K个节点即为要求的数。

    我的AC代码：

public class KthSmallestElementinaBST {
	Integer r = null;
	Integer idx = 0;

	public int kthSmallest(TreeNode root, int k) {
		idx = 0;
		travel(root, k);
		return r.intValue();
	}
	
	void travel(TreeNode root, int k ) {
		if(root == null && r != null) return;
		if(root.left != null)  travel(root.left, k);;
		if(++idx == k) r = root.val;
		if(root.right != null) travel(root.right, k);
	}
}

// Definition for a binary tree node.
class TreeNode {

	int			val;
	TreeNode	left;
	TreeNode	right;

	TreeNode(int x) {
		val = x;
	}
}