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: 1Example 2:
Input: root = [5,3,6,2,4,null,null,1], k = 3
5
/ \
3 6
/ \
2 4
/
1
Output: 3Follow 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);
}
}