Recently interviewed with Paytm for SDE3/backend role. Sharing the interview process and some questions asked.

Interview Process:

1. DSA Round
2. System Design (LLD + HLD) + DSA
3. Techno Managerial Round (System Design + DSA + Behavioural)

Round 1 — DSA

Topics asked:

• Trees
• Graphs
• Arrays

Question 1 — Subset Sum

Given an array of positive integers and a target, determine whether there exists a subset whose sum equals the target.

If found, print any valid subset.
Otherwise print:
"No subset found"

Example:

arr = [3, 34, 4, 12, 5, 2]
target = 9

Main concepts:

• Backtracking
• DP subset sum
• Recursion

Question 2 — Spring Transactional Behavior

Asked to explain:

• What this code does
• How @Transactional behaves
• What gets committed/rolled back

Code:

@Service
public class OrderService {
    @Transactional
    public void placeOrder() {
        saveOrder();
        sendNotification();
    }

    @Transactional
    public void saveOrder() {
        // persists order to the database
    }

    public void sendNotification() {
        throw new RuntimeException("Email service down");
    }
}

Main discussion points:

• Transaction boundaries
• Proxy-based AOP
• Internal method call issue
• Rollback behavior for RuntimeException
• Whether saveOrder() transactional annotation is effective during self-invocation

Round 2 — System Design + DSA

Combination of:

• LLD
• HLD
• Coding discussions

Focus areas:

• Scalability
• API design
• Concurrency
• Database design
• Tradeoffs

Round 3 — Techno Managerial

Topics:

• Previous project discussions
• System design follow-ups
• Behavioural questions
• Team handling
• Production issues
• Design tradeoffs
• DSA follow-ups

Overall interview focus:

• Backend engineering fundamentals
• Problem solving
• Distributed systems understanding
• Practical engineering decision making

Hope this helps others preparing for Paytm backend/SDE3 interviews.