230. Kth Smallest Element in a BST

Medium
Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.
Example 1:
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Input: root = [3,1,4,null,2], k = 1
Output: 1
Example 2:
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Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3
Constraints:
  • The number of nodes in the tree is n.
  • 1 <= k <= n <= 104
  • 0 <= Node.val <= 104
Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

解題

/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func kthSmallest(root *TreeNode, k int) int {
ans := 0
​
var helper func(*TreeNode)
helper = func(root *TreeNode){
if root == nil {
return
}
​
helper(root.Left)
​
k--
if k == 0 {
ans = root.Val
}
​
helper(root.Right)
}
​
helper(root)
​
return ans
}