Height of Binary Tree in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the height of this binary tree?

Consider the following binary tree structure and code to calculate its height. What is the output printed by the code?

DSA Javascript

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

function height(root) {
  if (!root) return 0;
  const leftHeight = height(root.left);
  const rightHeight = height(root.right);
  return Math.max(leftHeight, rightHeight) + 1;
}

const root = new Node(1);
root.left = new Node(2);
root.right = new Node(3);
root.left.left = new Node(4);
root.left.right = new Node(5);

console.log(height(root));

A 3

B 4

C 2

D 5

💡 Hint

Think about the longest path from the root to a leaf node.

The height is the number of nodes on the longest path from the root to a leaf. Here, the longest path is 1 -> 2 -> 4, which has 3 nodes, so height is 3.

❓ Predict Output

intermediate

Output of height function on a skewed tree

What will be the output of the height function when called on this skewed binary tree?

DSA Javascript

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

function height(root) {
  if (!root) return 0;
  return Math.max(height(root.left), height(root.right)) + 1;
}

const root = new Node(10);
root.right = new Node(20);
root.right.right = new Node(30);
root.right.right.right = new Node(40);

console.log(height(root));

A 0

B 3

C 1

D 4

💡 Hint

Count the nodes along the right side.

The tree is skewed to the right with 4 nodes in a line, so height is 4.

🔧 Debug

advanced

Identify the error in height calculation

The following code is intended to calculate the height of a binary tree. What error will it produce when run?

DSA Javascript

function height(root) {
  if (root == null) {
    return -1;
  }
  const leftHeight = height(root.left);
  const rightHeight = height(root.right);
  return Math.max(leftHeight, rightHeight);
}

const root = { value: 1, left: null, right: null };
console.log(height(root));

A 0

B TypeError

C -1

D Infinity

💡 Hint

Check the base case and the return statement.

The base case returns -1 for null nodes, but the function does not add 1 after max, so height of a single node returns -1 instead of 0.

🧠 Conceptual

advanced

Height of an empty tree

What is the height of an empty binary tree (null root)?

A 0

B -1

C 1

D undefined

💡 Hint

Think about how height is defined for no nodes.

An empty tree has height 0 because there are no nodes.

🚀 Application

expert

Calculate height after inserting nodes

Given an initially empty binary tree, nodes are inserted in this order: 5, 3, 8, 1, 4, 7, 9. What is the height of the tree after all insertions?

A 4

B 3

C 2

D 5

💡 Hint

Visualize the tree structure after inserting nodes in given order.

The tree is balanced with height 3 after inserting nodes in given order.

Practice

1. What does the height of a binary tree represent?

easy

A. The number of children of the root node

B. The total number of nodes in the tree

C. The number of leaf nodes in the tree

D. The number of edges on the longest path from root to a leaf

Solution

1. Step 1: Understand the definition of height in a binary tree

   The height is the longest path from the root node down to the farthest leaf node, counted by edges.

2. Step 2: Differentiate height from other tree properties

   Number of nodes or leaf nodes are different concepts; height specifically counts edges on the longest path.

3. Final Answer:

   The number of edges on the longest path from root to a leaf -> Option D

4. Quick Check:

   Height = longest root-to-leaf edge count [OK]

Hint: Height counts edges, not nodes, on longest root-to-leaf path [OK]

Common Mistakes:

• Confusing height with total nodes count
• Counting nodes instead of edges for height
• Thinking height equals number of leaf nodes

2. Which of the following is the correct base case to find the height of a binary tree recursively in JavaScript?

easy

A. if (node === null) return -1;

B. if (node === null) return 0;

C. if (node === undefined) return 1;

D. if (node === undefined) return 0;

Solution

1. Step 1: Understand the base case for empty subtree

   When the node is null (no node), height is defined as -1 to count edges properly.

2. Step 2: Why -1 instead of 0?

   Returning -1 for null ensures leaf nodes have height 0 (0 edges below them), matching edge count definition.

3. Final Answer:

   if (node === null) return -1; -> Option A

4. Quick Check:

   Empty node height = -1 for correct edge count [OK]

Hint: Use -1 for null node to count edges correctly [OK]

Common Mistakes:

• Returning 0 for null node causes off-by-one height
• Checking undefined instead of null for nodes
• Confusing node absence with leaf node

3. What is the output of the following code snippet?

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

function height(node) {
  if (node === null) return -1;
  return 1 + Math.max(height(node.left), height(node.right));
}

const root = new Node(10);
root.left = new Node(5);
root.right = new Node(15);
root.left.left = new Node(3);
root.left.left.left = new Node(1);

console.log(height(root));

medium

A. 5

B. 3

C. 4

D. 2

Solution

1. Step 1: Trace the tree structure and count edges

   The longest path is from root (10) to left-left-left (1): edges are 10->5, 5->3, 3->1, total 3 edges.

2. Step 2: Calculate height using the function logic

   Function returns 1 + max height of children. For leaf node (1), height = 0. So total height = 3 edges means function returns 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Longest path edges = 3 [OK]

Hint: Count edges on longest path, not nodes [OK]

Common Mistakes:

• Counting nodes instead of edges for height
• Misreading tree structure and missing deepest node
• Confusing return value for leaf node height

4. Find the error in this height function and select the correct fix:

function height(node) {
  if (node === null) return 0;
  return Math.max(height(node.left), height(node.right));
}

medium

A. Change base case to return -1 instead of 0

B. Return sum instead of max of left and right heights

C. Add +1 before Math.max to count current node

D. No error, function is correct

Solution

1. Step 1: Analyze the base case and recursive return

   Base case returns 0 for null, which is acceptable if counting nodes, but function misses adding 1 for current node.

2. Step 2: Identify missing increment for current node

   Function returns max height of children but does not add 1 for current node, so height is underestimated by 1.

3. Final Answer:

   Add +1 before Math.max to count current node -> Option C

4. Quick Check:

   Height = 1 + max(child heights) [OK]

Hint: Add +1 to include current node in height [OK]

Common Mistakes:

• Forgetting to add 1 for current node
• Changing base case unnecessarily
• Returning sum instead of max height

5. Given a binary tree where some nodes have only one child, how does the height calculation change if you want to count the number of nodes on the longest path instead of edges? Choose the correct modification to the recursive height function.

function height(node) {
  if (node === null) return ?;
  return ? + Math.max(height(node.left), height(node.right));
}

hard

A. Return 0 for null; add 1 before Math.max to count nodes

B. Return -1 for null; add 1 before Math.max to count nodes

C. Return 1 for null; add 0 before Math.max to count nodes

D. Return 0 for null; add 0 before Math.max to count nodes

Solution

1. Step 1: Understand difference between counting edges and nodes

   Counting nodes means height of empty tree is 0, and each node adds 1 to path length.

2. Step 2: Adjust base case and recursive addition accordingly

   Return 0 for null node (no nodes), and add 1 for current node to count nodes on path.

3. Final Answer:

   Return 0 for null; add 1 before Math.max to count nodes -> Option A

4. Quick Check:

   Height counts nodes: base=0, add 1 per node [OK]

Hint: Use 0 base and add 1 to count nodes, not edges [OK]

Common Mistakes:

• Using -1 base for node count causes off-by-one
• Not adding 1 for current node
• Confusing edges count with nodes count