Paytm | SSE (Backend) Interview Experience | Rejected

1. Online Assessment (OA)

• Duration: 60 minutes
• Format:
  • 2 DSA problems (LeetCode Medium level)
  • MCQs on core CS fundamentals (DSA, OOPs, DB, OS basics)

2. DSA + Low-Level Design Round

This round started with discussion and gradually moved into problem-solving and design.

a. Project Discussion

• Interviewer asked about one of my past projects
• Then asked me to choose another project and explain:
  • Problem statement
  • Design decisions
  • Challenges and trade-offs

b. DSA Questions

1. Check if two binary trees are identical
   • Asked to explain the approach verbally
   • Focus was on recursion, base cases, and time complexity
2. Rotten Oranges problem
   • Explained the BFS-based algorithm
   • Wrote clean, working code
   • Discussed time and space complexity

c. System Design (Twitter-like System)

Core Requirements:

• Follow / Unfollow users
• Post a tweet
• Show user feed

Discussion Points:

• High-level architecture
• Scalability considerations
• Feed generation approach (fan-out strategies)
• Database schema design

Extension: Search Tweets by Content

• Started with multiple approaches:
  • SQL-based search (limitations at scale)
  • In-memory indexing
• Discussed drawbacks of each
• Final approach:
  • Using a search engine (e.g., inverted index)
  • DB choice and justification for search use case

3. System Design Round

a. Design a Job Scheduler System

Problem Statement:

• System runs every day at a fixed time window
• Executes scheduled jobs during this window

Key Requirements:

• Scalability
• Reliability
• Exactly-once notification delivery

b. Code Review Question

• Given a code snippet
• Asked to identify:
  • Logical issues
  • Concurrency or reliability problems
  • Possible improvements

Final Verdict: Rejected