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Grokking the Coding Interview Patterns in Python

Kth Smallest Element in a BST

Explore how to identify the kth smallest element in a binary search tree using depth-first search. Understand the problem constraints, practice restructuring logic, and implement an optimal solution. This lesson enhances your ability to solve common tree-based coding interview questions efficiently.

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Statement

Given the root node of a binary search tree and an integer value k, return the kthk^{th} smallest value in the tree.

Constraints:

  • The number of nodes in the tree is nn.
  • 1≤k≤n≤5001 \leq k \leq n \leq 500
  • 0≤0 \leq Node.data ≤104\leq 10^4

Examples

Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Kth Smallest Element in a BST

1.

What is the kthk^{th} smallest element when we have the following input?

k = 4

BST:

   3
  / \
 1   4
  \ 
   2
A.

4

B.

1

C.

3

D.

2

1 / 2

Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Note: As an additional challenge, we have intentionally hidden the solution to this puzzle.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
1
2
3
4

Try it yourself

Implement your solution in main.py in the following coding playground.

We have left the solution to this challenge as an exercise for you. An optimal solution to this problem runs in O(n) time and takes O(n) space. You may try to translate the logic of the solved puzzle into a coded solution.

Python
usercode > main.py
# Definition for a binary tree node

# class TreeNode:

#     def __init__(self, data):

#         self.data = data

#         self.left = None

#         self.right = None

from ds_v1.BinaryTree.BinaryTree import TreeNode



def kth_smallest_element(root, k):



    # Replace this placeholder return statement with your code

    return -1
Kth Smallest Element in a BST