kth_smallest_element_in_bst.py

"""
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?


"""

# 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:
        values = []
        self.dfs(root, values)
        return values[k - 1]

    def dfs(self, root, values):
        if root is None:
            return
        self.dfs(root.left, values)
        values.append(root.val)
        self.dfs(root.right, values)