Paytm Interviews

This was my interview experience for the SDE Java Backend role at Paytm. The first round was heavily focused on DSA. The interviewer started by gracefully asking about my background and work experience, including my tech stack. Immediately after, they posed the first question: "Longest Substring Without Repeating Characters".

I was able to solve it and thoroughly explained my approach to the interviewer, who was satisfied with my code and explanation.

Following that, the second question was "Combination Sum". Initially, I felt a wave of panic, but I managed to compose myself and asked the interviewer for clarification on the problem's demands. They were very helpful, providing hints throughout. Through brainstorming, I was able to crack the problem and proceed to write the code. The moment I completed the base case for the backtracking algorithm, the interviewer seemed confident in my ability to finish it.

The interview then concluded. The verdict was positive: I was selected for Round 2, which will be a System Design round.

I recently interviewed for the Golang Developer role at Paytm. I am currently an SDE 2 at a product-based company with over 3 years of experience. The role I applied for required similar experience (3+ years SDE). The entire process was stringent about using Golang for all coding and design tasks.

The first round focused on Data Structures and Algorithms. I was asked two medium-level DSA questions.

The second round was a Low-Level Design (LLD) round. I was asked to design a user management system. This round required setting up a local editor and utilizing specific MySQL/GORM packages, which was an interesting constraint. There was also a discussion about my past projects.

The third round was quite challenging. It involved three DSA questions, ranging from medium to hard difficulty, which had to be solved in an online editor. Similar to the second round, there was a detailed discussion about my projects. Additionally, this round delved into deeper system concepts such as threads, Kafka's workings, and identifying memory leaks.

Regrettably, I was not able to clear the third round. The compensation offered for this role was 29LPA base with an additional 1L bonus.

I came across a Google form on LinkedIn and decided to apply. After 1-2 weeks, I received a call from a recruiter who explained the role and process, then scheduled my first interview and shared details about an upcoming online assessment.

Interview Rounds

Round 1 – Technical + DSA

This round started with introductions and a deep dive into my current projects. We discussed my contributions, the technology stack I used, and my problem-solving approaches. For DSA, I was given a basic Linked List problem, where I discussed multiple approaches and optimizations. Java questions focused on Lambdas and Stream APIs. I received strong positive feedback from HR an hour later, and the next round was scheduled.

Round 2 – Online Assessment (Hackerrank)

I completed both questions successfully. One was an easy-level problem which I don't recall the exact details of, and the other was the classic Longest Common Substring problem, which I solved completely.

Round 3 – Core Java, Design Patterns & System Design

This round began with detailed project discussions, including my technology choices and the reasoning behind them. Topics covered included Design Patterns, various Spring Boot concepts, and authentication mechanisms. I felt I could have explained authentication better. We also went through some SQL queries. The next day, HR informed me that the feedback was positive and scheduled the final round.

Round 4 – High-Level System Design

This discussion focused on my major contributions in my current role. For the HLD question, I was asked to design a system similar to Shiprocket, with a particular emphasis on order management and payment processing. The interviewer was very professional and supportive, making it a smooth experience.

Final Outcome

After 2-3 days, HR informed me that I had cleared all rounds and shared the offer letter. However, I ultimately declined the offer as PayTm has a Work From Office (WFO) policy, and I prefer Work From Home (WFH).

I recently interviewed at Paytm for the Senior Technical Lead role. Sharing my Round 1 experience for those preparing for similar senior-level interviews.

Round 1 – DSA + Behavioural

This round focused on a combination of data structures/algorithms and behavioural evaluation.

DSA Questions:

The interviewer first asked about my proficiency level in DSA, and based on that, started with the Trap Rain Water problem.

I was able to come up with a working solution after some discussion. The interviewer asked me to code it, but since the implementation was taking longer than expected, we skipped the full code and moved on.

The next problem was Left View of a Binary Tree.

I was able to explain the logic quickly and successfully implemented the code during the interview.

Behavioural Questions:

The second half of the round focused on behavioural questions using the STAR format.

Key questions included:

• A situation where I worked on improving response time and system efficiency significantly.
• A project or moment where I was regarded exceptionally well by peers or leadership.

This round was designed to assess both problem-solving ability and leadership mindset — crucial for any senior-level engineering role

Round 2 - Technical cum System Design

After introduction started with general Java questions (Collections framework) and tricky output based questions. Asked some questions related to overriding, static, try, finally, multiple inheritance.

Then asked some questions related to Spring Boot.

Then asked to design the system for view count and like count of videos.

Also asked about how the video streaming system works, how streaming switches to low video quality.

🚀 During my recent backend interviews, I encountered some intriguing questions I came across. Here's some of them :

☕ Java + Spring Boot Q: How do you configure thread pools in Spring Boot to ensure efficient task execution under high load? ✅ Used ThreadPoolTaskExecutor to manage thread pools. ✅ Tuned corePoolSize and maxPoolSize based on whether tasks were CPU-bound or IO-bound. ✅ Adjusted keepAliveTime to recycle idle threads and reduce resource usage.

🔁 System Recovery with Backlog Processing Q: Your app handles 100 RPS and each task takes 100ms. A 5-minute outage means 30K pending tasks. Once restored, how do you process backlog + new incoming traffic in 5 minutes? ✅ Needed to support 200 RPS (100 backlog + 100 new). ✅ Designed executor with sufficient thread pool and queue capacity. ✅ Discussed scaling strategies: using separate executors to meet deadlines.

🧩 Singleton Injecting a Request-Scoped Bean Q: Why does injecting a @RequestScope bean into a singleton fail in Spring Boot? ✅ Request-scoped beans are instantiated per HTTP request, but singleton beans are created at app startup. 🔧 Fixes: Used ObjectFactory, Provider, or enabled proxying for lazy injection.

🔄 @RefreshScope in Spring Cloud Q: What is the role of @RefreshScope in dynamic configuration updates? ✅ Allows runtime refresh of beans when config values change in Spring Cloud Config. ✅ Works with Spring Cloud Bus to auto-propagate config changes—no app restart needed.

📩 Kafka Q: If a Kafka consumer fails to process a message, how do you retry it 5 times and eventually discard it? ✅ Built retry logic using separate retry topics with increasing backoff. ✅ After 5 attempts, failed messages are sent to a Dead Letter Queue (DLQ) for logging or manual handling. ✅ Ensures fault-tolerance without losing messages silently.

🧱 LLD – Canvas Fill Tool 🖌️ Design an MS Paint-like canvas fill backend: Q: Users can draw closed shapes. If they click inside, the entire shape should be filled. Clicking outside fills only that pixel. ✅ Tracked shape boundaries. ✅ Used Flood Fill Algorithm (DFS/BFS) to propagate fill from clicked pixel until boundaries are hit. ✅ Ensured scalability for large canvases and concurrent fill operations.

⚙️ Infra & System Design 📦 PostgreSQL Internals Q: How does PostgreSQL optimize query execution and data retrieval? ✅ Discussed B-Tree and GIN indexes.

🌐 Kubernetes Service Discovery Q: Pod IPs change frequently. How do services reliably discover and communicate with each other? ✅ Kubernetes assigns a stable ClusterIP to each service.

📬 Designing a Pub/Sub System Q: How do you implement a loosely-coupled publisher-subscriber architecture? ✅ Used Observer Pattern, Event Bus libraries, or message brokers like Kafka/RabbitMQ.

Round 1: DSA Online Assessment

a. Given an array and a window size k, find the first negative integer in every window. (Sliding Window Technique)

b. Implement an LRU (Least Recently Used) Cache with efficient get and put operations using suitable data structures.

Round 2: Low-Level Design

a. Design a UPI-like system supporting:

• Multiple types of wallets (e.g., Paytm Wallet, Fastag, etc.)
• A main bank account linked to the user

Required to define proper data models and implement core functionalities like add money, transfer, balance check, and transaction history.

Round 3: System Design

a. Concurrency Control:

• Discussed database locking mechanisms (pessimistic vs optimistic)
• Explained transaction isolation levels: Read Uncommitted, Read Committed, Repeatable Read, and Serializable with examples.

b. Distributed Caching:

• In-depth discussion on Redis – data structures, TTL, eviction policies, and use-cases
• Explained caching strategies: write-through, write-behind, and cache-aside.

c. Handling Thread Downtime:

• Was asked how I would manage thread failures or downtime under high-load or continuous request scenarios
• Discussed approaches like thread pools, retries, circuit breakers, and fallback mechanisms to ensure system resilience.

Verdict: Selected

Although I didn’t join, it was a great experience and a solid learning opportunity.

I recently interviewed for the Senior Software Engineer (Backend) position at Paytm Money. The entire recruitment process spanned approximately two weeks, from the initial Online Assessment to the final interview round.

Round 1: Online Assessment (OA)
This round lasted 63 minutes and consisted of two coding questions, which I found to be of medium-easy difficulty, along with six Multiple Choice Questions focusing on DSA theory, particularly around tree data structures. I successfully passed this round.

Round 2: DSA Interview
This hour-long interview focused on Data Structures and Algorithms. The interviewer posed two specific coding challenges:
1. Unique Paths (Dynamic Programming)
2. Search in Rotated Sorted Array (Binary Search)
Beyond the coding problems, we also delved into several backend-related topics, including Spring Boot basics and annotations, the lifecycle and working principles of the Garbage Collector, and features and lifecycle of the Stream API. I cleared this round as well.

Round 3: HM / System Design Interview
This was the final round, lasting one hour. It primarily involved an in-depth discussion about my past projects, where we covered their impact, trade-offs made, architectural decisions, testing strategies, and potential future improvements. I was also questioned on HTTP Status Codes, specifically understanding 4xx (client errors) and 3xx (redirection) categories. The interviewer then presented a coding challenge: 'Find the Kth Node from the End in a Linked List'. Following this, there was a threading question: 'Using two threads, print numbers from two arrays (odd and even) in sequence, for example: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10'.

Unfortunately, despite my efforts, I did not receive an offer letter after this final round.

I recently interviewed for the SSE role at Paytm. The interview process consisted of three rounds.

The first round was an online test conducted on HackerRank. This round included two easy LeetCode problems, one medium/hard problem, and multiple choice questions covering various technical concepts.

The second round focused on Data Structures and Algorithms. During this round, I was asked to solve a variation of Kadane's algorithm, to implement a solution to detect and break a cycle in a linked list, and to find the maximum sum path in a binary tree. Additionally, the interviewer asked several questions about my current projects and their technical details.

The third and final round was a System Design interview. I was presented with the challenge of designing an elevator system, requiring me to consider aspects like multi-elevator control, request handling, and fault tolerance. This round also involved scenario-based questions that tested my understanding of crucial system design principles such as caching, idempotency, data consistency, and strategies for scaling distributed systems.