LeetCode-in-All

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 <= 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?

Solution

from typing import Optional

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.k = 0
        self.count = 0
        self.val = None

    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        self.k = k
        self.count = 0
        self.val = None
        self.calculate(root)
        return self.val

    def calculate(self, node):
        if not node:
            return

        if node.left:
            self.calculate(node.left)

        self.count += 1
        if self.count == self.k:
            self.val = node.val
            return

        if node.right:
            self.calculate(node.right)

This site is open source. Improve this page.