kth-smallest-element-in-a-bst.py

from typing import Optional


# 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 kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        nodes = []

        def inorder(node: TreeNode):
            if not node:
                return

            inorder(node.left)
            nodes.append(node.val)
            inorder(node.right)

        inorder(root)
        return nodes[k - 1]