Gullak SDE 1 Interview Experience

I recently interviewed with Gullak for an SDE-1 Backend position, and here's how the process went:

Round 1 – Online Assessment (OA)

• MCQs
• 2 DSA questions (medium difficulty, in my opinion)
• Platform: Equip

Round 2 – Problem Solving Round

I went in expecting a DSA-heavy interview, but it turned out to be more focused on logical reasoning, problem-solving, and system design.

Problem 1

In a class assessment, the average score is calculated as x.

• Student A's actual score: 100, recorded score: 0
• After correcting this mistake, the average becomes x + 4

There's another mistake:

• Student B's actual score: 56, recorded score: 81

After correcting this second mistake as well, the average becomes y.

Question: Find y − x.

Problem 2

There are 100 people and 100 lockers, both numbered from 1 to 100.

• Initially, all lockers are closed.
• Person 1 toggles every locker.
• Person 2 toggles lockers numbered 2, 4, 6, ...
• Person 3 toggles lockers numbered 3, 6, 9, ...
• And so on.

Each person toggles all lockers that are multiples of their number.

Question: After all 100 people have finished, which lockers remain open and which remain closed?

Problem 3 – System Design

Design a QR-code-based login system where:

• A web browser displays a QR code.
• A logged-in mobile app scans the QR code.
• The user gets authenticated on the web without entering an OTP or password.

Discussion areas:

• Data models
• API design
• Authentication flow
• Concurrency handling
• Locking / wait-nowait mechanisms
• Edge cases (e.g., two devices scanning the same QR code simultaneously)

Result: Awaiting feedback.