BST Find Minimum Element in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find minimum element in a BST

What is the output of the following code that finds the minimum element in a Binary Search Tree (BST)?

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

function findMin(root) {
  let current = root;
  while (current.left !== null) {
    current = current.left;
  }
  return current.value;
}

const root = new Node(10);
root.left = new Node(5);
root.right = new Node(15);
root.left.left = new Node(2);
root.left.right = new Node(7);
root.right.right = new Node(20);

console.log(findMin(root));

A 5

B 7

C 2

D 10

💡 Hint

The minimum element in a BST is the leftmost node.

In a BST, the smallest value is found by going as far left as possible from the root. Here, the leftmost node has value 2.

❓ Predict Output

intermediate

Find minimum in BST with single node

What will the output be when finding the minimum element in a BST with only one node?

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

function findMin(root) {
  let current = root;
  while (current.left !== null) {
    current = current.left;
  }
  return current.value;
}

const root = new Node(42);

console.log(findMin(root));

A 42

B null

C undefined

D Error

💡 Hint

If there is only one node, that node is the minimum.

Since the root has no left child, the loop does not run and the root's value (42) is returned.

❓ Predict Output

advanced

Find minimum element in BST with null root

What error or output will this code produce when trying to find the minimum element in an empty BST (root is null)?

DSA Javascript

function findMin(root) {
  let current = root;
  while (current.left !== null) {
    current = current.left;
  }
  return current.value;
}

const root = null;

console.log(findMin(root));

A null

B TypeError: Cannot read property 'left' of null

C undefined

D 0

💡 Hint

Check what happens when root is null and you try to access root.left.

Since root is null, current is null. Accessing current.left causes a TypeError.

🧠 Conceptual

advanced

Why does findMin in BST only check left children?

Why does the function to find the minimum element in a BST only traverse left children and not right children?

A Because right children are always larger, so checking them is unnecessary for minimum.

B Because the minimum could be anywhere, but left is checked first by convention.

C Because the function is incorrect and should check both sides.

D Because in a BST, all left children are smaller than their parent nodes, so the minimum is found by going left.

💡 Hint

Think about the BST property for left and right children.

In a BST, left children have smaller values than their parents, so the smallest value is always found by moving left until no more left child exists.

❓ Predict Output

expert

Find minimum element after BST modification

Given the BST below, what is the output after inserting a new node with value 1 and then finding the minimum element?

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

function insert(root, value) {
  if (root === null) return new Node(value);
  if (value < root.value) {
    root.left = insert(root.left, value);
  } else {
    root.right = insert(root.right, value);
  }
  return root;
}

function findMin(root) {
  let current = root;
  while (current.left !== null) {
    current = current.left;
  }
  return current.value;
}

const root = new Node(10);
root.left = new Node(5);
root.right = new Node(15);
root.left.left = new Node(2);
root.left.right = new Node(7);
root.right.right = new Node(20);

insert(root, 1);

console.log(findMin(root));

A 1

B 2

C 5

D 10

💡 Hint

After inserting 1, it becomes the new leftmost node.

The new node with value 1 is inserted as the left child of 2, making 1 the smallest value in the BST.