[Solved] 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.

Question

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?

Python 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
def inorder(root,res,k):
    if root==None:return
    heapq.heappush(res,-root.val)
    if len(res)>k:heapq.heappop(res)
    inorder(root.left,res,k)
    inorder(root.right,res,k)

class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        res = []
        heapq.heapify(res)
        inorder(root,res,k)
        return -res[0]