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Find the Kth Smallest Element in a BST - Mastering Tree Algorithms for Python Coding Interviews

Updated: at 04:34 AM

Binary search trees (BSTs) are a fundamental data structure in computer science, used to store data in a way that allows for efficient searching, insertion, and deletion. A key component of working with BSTs is being able to retrieve elements from the tree in a specific order. One common task is finding the kth smallest element in a BST.

This how-to guide will provide a comprehensive overview of techniques for finding the kth smallest element in a BST using Python. We will cover the basics of BSTs, formalize the problem statement, present both recursive and iterative solutions with annotated example code, analyze the time and space complexities of each algorithm, and discuss optimizations and applications.

Follow along to gain a solid understanding of this key technical interview question and learn how to implement elegant solutions in Python. The concepts and code examples provided will build your skills in writing clean, efficient algorithms leveraging data structures like binary trees. Let’s get started!

Table of Contents

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Overview of Binary Search Trees

A binary search tree (BST) is a hierarchical data structure that consists of nodes, each with at most two child nodes - a left child and a right child. BSTs have the following key properties:

This ordering of keys allows binary search trees to support efficient searching, insertion, and deletion in O(log n) time on average. BSTs are commonly used to implement associative arrays and sets.

Here is an example of constructing a simple BST in Python:

class Node:
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None

def insert(root, key):
    if root is None:
        return Node(key)
        if key < root.key:
            root.left = insert(root.left, key)
            root.right = insert(root.right, key)
    return root

root = None
root = insert(root, 15)
root = insert(root, 10)
root = insert(root, 20)
root = insert(root, 12)
root = insert(root, 30)

This code first defines a Node class representing each node in the tree. The insert() function recursively inserts a new node in the correct position to maintain the BST ordering. Given this sample BST:


10 20 / \ /
5 12 17 30

The key 15 is at the root, with left subtree keys less than 15 and right subtree keys greater than 15. This structure allows efficient lookup, insertion, and deletion.

Now let’s formalize the problem we want to solve: finding the kth smallest element in a binary search tree.

Problem Statement

Given a binary search tree and a number k, find the kth smallest element in the BST.


Input:BST root = [15, 10, 20, 5, 12, 17, 30], k = 3 Output: 12

The 3rd smallest element in the BST is 12.

A brute force approach would be to invert the tree to get all elements in ascending order, store them in an array, and return the element at index k-1. However, this is inefficient, requiring O(N) time and O(N) space to store the array.

We can do better by utilizing the BST structure. Let’s walk through two optimal solutions - a recursive approach and an iterative approach.

Recursive Solution

We can find the kth smallest element recursively by keeping track of the size of the left subtree at each node. The key steps are:

  1. Recursively find the size of the left subtree.
  2. If the size = k-1, we have found the kth smallest element.
  3. Otherwise, if the size > k-1, continue recursively searching just the left subtree.
  4. Else, recurse on the right subtree for the (k - size - 1)th smallest element.

Here is an implementation in Python:

class Node:
  def __init__(self, val):
    self.val = val
    self.left = None
    self.right = None

def kthSmallest(root, k):
  # Base case
  if root is None:
    return None

  # Recursively get size of left subtree
  left_size = getSize(root.left)

  # If left subtree has k-1 nodes, root is k-th smallest
  if left_size == k - 1:
    return root.val

  # If left subtree has more than k-1 nodes,
  # search just the left subtree
  elif left_size > k - 1:
    return kthSmallest(root.left, k)

  # Otherwise, search right subtree for
  # (k - size of left subtree - 1)th smallest element
    return kthSmallest(root.right, k - left_size - 1)

# Recursively get size of left subtree
def getSize(node):
  if node is None:
    return 0
  return 1 + getSize(node.left) + getSize(node.right)


  1. Base case returns None if the tree is empty.

  2. Recursively compute the size of the left subtree using the getSize() helper.

  3. Compare the left subtree size to k-1 to determine which subtree to recurse on.

  4. If left size is k-1, the root is the kth smallest element.

  5. If left size is greater than k-1, only recurse on left subtree.

  6. Otherwise, recurse on right subtree, decrementing k by the left size + 1 for root node itself.

This elegantly handles all cases by exploiting the BST ordering and without needing extra storage!

Time Complexity: O(H + k), where H is height of BST. We traverse at most k nodes in the recursive calls.

Space Complexity: O(H) to keep recursion stack.

Iterative Solution

We can also solve this problem iteratively using a stack. The steps are:

  1. Initialize current node as root

  2. Push root and all left descendants onto stack while traversing the leftmost branch

  3. Pop a node from stack. If popped node matches k, return it.

  4. Increment count of visited nodes

  5. Add right child of popped node to stack

  6. Repeat steps 3-5 until count = k.

Python implementation:

class Node:
  def __init__(self, val):
    self.val = val
    self.left = None
    self.right = None

def kthSmallest(root, k):
  stack = []

  # Push root and all left descendants
  curr = root
  while curr:
    curr = curr.left

  # Iteratively pop from stack
  i = 0
  while stack:
    curr = stack.pop()

    # If popped node is k-th element, return it
    i += 1
    if i == k:
      return curr.val

    # Push right child onto stack and repeat
    if curr.right:

  return None


  1. Push the root node and all left descendants onto the stack to reach the leftmost leaf.

  2. Pop nodes off the stack, incrementing a counter i.

  3. When i = k, the popped node is the kth smallest element.

  4. After popping a node, push its right child to traverse back up the tree.

This iterative approach also handles all cases in O(H+k) time without recursion!

Time Complexity: O(H + k)

Space Complexity: O(H) to store stack

The iterative solution is generally preferred in interviews for its efficiency and being easier to reason about.

Optimizations and Analysis

Balanced vs Unbalanced BSTs:

The time complexities above are for balanced BSTs with minimal height. In unbalanced BSTs, the worst case height could be O(N) leading to O(N+k) time complexity.

To guarantee O(H+k) time, use a self-balancing BST like AVL or Red-Black trees. The height is O(log N), so time would be O(log N + k).

Average Case:

For a randomly built BST where each node has equal probability of being the kth smallest, the average case time complexity is O(log N + k).

Early Exit Optimization:

We can optimize both solutions by early exiting once k nodes have been visited. No need to fully traverse or compute subtree sizes.

Applications and Analysis

Finding the kth smallest element has many useful applications:

The techniques covered to solve this problem demonstrate important skills:

This question is common in coding interviews since it combines data structures, recursion, and algorithm analysis. Mastering both recursive and iterative approaches shows strong technical and analytical abilities.


Finding the kth smallest element in a Binary Search Tree is a key problem to assess candidate skills in coding interviews. We covered how to solve it in Python elegantly using both recursion and iteration.

The techniques demonstrated - leveraging the BST structure, recursive reasoning, iterative stacks, and analyzing time/space complexity - are applicable across domains like databases, search engines, and statistics.

Practice implementing both solutions from scratch until you have mastered this pattern. Knowing advanced data structures like self-balancing BSTs also helps optimize performance.

I hope you found this guide helpful! Let me know if you have any other questions on mastering technical interview algorithms.