Classplus Software Engineer Interview: Rounds, Questions and Preparation

What the job involves

Software Engineers at Indian unicorns and high-growth startups like Classplus build and scale consumer-facing and internal products, often working across the stack to ship features quickly. Day to day involves writing clean code, debugging live issues, collaborating with product and design, and optimising for performance and reliability in a fast-paced environment.

Role and Technical Knowledge

1. 1

   Explain how you would design a scalable notification system for a learning app that sends reminders to millions of students.

   Cover message queues, push vs pull, batching, retries, and handling peak loads during exam seasons.

2. 2

   What is the difference between SQL and NoSQL databases? When would you choose one over the other for a user profile service?

   Discuss schema flexibility, consistency, scaling, and give a concrete example like user profiles needing frequent reads.

3. 3

   How does a REST API work? What are idempotent methods and why do they matter in payment flows?

   Explain HTTP methods, status codes, and why idempotency prevents double charges in transactions.

4. 4

   Given a list of student scores, write a function to find the top K performers efficiently.

   Use a min-heap of size K for O(n log K) time, and discuss edge cases like ties and empty input.

Practical Scenarios

1. 5

   A feature you deployed is causing slow page loads for users in a specific region. How do you debug and fix it?

   Check CDN, server logs, latency metrics, and regional infrastructure; propose caching or edge deployment.

2. 6

   You need to add a new payment gateway to an existing ed-tech platform. What steps would you take from planning to release?

   Outline API integration, sandbox testing, security checks, rollback plan, and monitoring.

3. 7

   How would you handle a situation where your code passes tests locally but fails in production?

   Mention environment differences, config management, logging, and using staging environments to reproduce.

4. 8

   You are asked to reduce the load time of a dashboard that fetches data from multiple APIs. What optimisations would you apply?

   Discuss parallel requests, caching, pagination, lazy loading, and database indexing.

Behavioural and HR

1. 9

   Tell me about a time you had to learn a new technology quickly for a project. How did you approach it?

   Use STAR method: describe the situation, your learning strategy, and the outcome with a measurable result.

2. 10

   Describe a situation where you disagreed with a teammate on a technical approach. How did you resolve it?

   Show empathy, data-driven discussion, and willingness to compromise for the team's goal.

3. 11

   Why do you want to work at a high-growth startup like Classplus instead of a large MNC?

   Highlight ownership, fast-paced learning, impact, and comfort with ambiguity.

4. 12

   How do you prioritise tasks when you have multiple deadlines and limited time?

   Mention urgency vs importance, communication with stakeholders, and using tools like Trello or Jira.

Problem Solving and Coding

1. 13

   Given an array of integers, find the longest subarray with sum equal to zero.

   Use a hashmap to store prefix sums and their first occurrence; time complexity O(n).

2. 14

   Write a function to detect a cycle in a linked list. Explain your approach.

   Use Floyd's cycle detection (tortoise and hare) with two pointers; discuss O(n) time and O(1) space.

3. 15

   How would you find the first non-repeating character in a string?

   Use a hashmap to count frequencies, then iterate to find the first with count 1; O(n) time.

4. 16

   Given a binary tree, print its nodes level by level.

   Use BFS with a queue; mention handling null children and tracking levels.