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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:
Input: root = [3,1,4,null,2], k = 1
Output: 1
Example 2:
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 <= 1040 <= 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?
Solution
# Definition for a binary tree node.
# class TreeNode
# attr_accessor :val, :left, :right
# def initialize(val = 0, left = nil, right = nil)
# @val = val
# @left = left
# @right = right
# end
# end
# @param {TreeNode} root
# @param {Integer} k
# @return {Integer}
def kth_smallest(root, k)
@k = k
@count = 0
@val = 0
calculate(root)
@val
end
private
def calculate(node)
return unless node
if node.left.nil? && node.right.nil?
@count += 1
@val = node.val if @count == @k
return
end
calculate(node.left) if node.left
@count += 1
@val = node.val if @count == @k
return if @count == @k
calculate(node.right) if node.right
end