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

# 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.