230. Kth Smallest Element in a BST #

Problem #

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

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?

Problem Summary #

Given a binary search tree, write a function kthSmallest to find the k th smallest element in it. You may assume k is always valid, 1 ≤ k ≤ the number of elements in the binary search tree.

Solution Idea #

• Because of the ordered property of a binary search tree, perform an inorder traversal on it; when the Kth number is reached, that is the result

Code #

package leetcode

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func kthSmallest(root *TreeNode, k int) int {
	res, count := 0, 0
	inorder230(root, k, &count, &res)
	return res
}

func inorder230(node *TreeNode, k int, count *int, ans *int) {
	if node != nil {
		inorder230(node.Left, k, count, ans)
		*count++
		if *count == k {
			*ans = node.Val
			return
		}
		inorder230(node.Right, k, count, ans)
	}
}