Height of Binary Tree in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the height of a binary tree?

The height of a binary tree is the number of edges on the longest path from the root node to a leaf node. It measures how tall the tree is.

beginner

How do you calculate the height of a binary tree recursively?

To calculate height recursively, find the height of left and right subtrees, then take the maximum of these two heights and add 1 for the current node.

beginner

What is the height of an empty binary tree?

The height of an empty binary tree is defined as -1 because there are no nodes and no edges.

intermediate

Why is height important in binary trees?

Height helps understand the tree's balance and efficiency. Smaller height means faster operations like search, insert, and delete.

beginner

What is the height of a binary tree with only one node (root)?

The height is 0 because there are no edges from the root to any leaf (root itself is the leaf).

What does the height of a binary tree represent?

A Number of edges on the longest path from root to leaf

B Number of nodes in the tree

C Number of leaf nodes

D Number of children of the root

Height counts edges on the longest path from root to leaf, not nodes or children count.

What is the height of an empty binary tree?

A 0

B 1

C Undefined

D -1

By definition, empty tree height is -1 because no nodes or edges exist.

How do you find the height of a binary tree recursively?

A Add heights of left and right subtrees

B Take maximum height of left and right subtrees and add 1

C Count all nodes in the tree

D Take minimum height of left and right subtrees

Height is max height of subtrees plus 1 for current node.

What is the height of a binary tree with only root node?

A 0

B 1

C -1

D 2

Only root means no edges, so height is 0.

Why is knowing the height of a binary tree useful?

A To find the number of leaf nodes

B To count total nodes

C To understand tree balance and operation efficiency

D To find the root node

Height helps assess tree balance and speed of operations.

Explain how to calculate the height of a binary tree using recursion.

Think about how you visit each node and combine subtree heights.

Describe why the height of a binary tree matters in practical use.

Consider how tall a tree is and how that affects work done.

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