Card91 interview experience Real time questions & tips from candidates to crack your interview
Backend engineer
Card91
3 rounds | 5 Coding
problems
Selected Interview preparation journey
Journey
I’ve been preparing for interviews for a while now, mostly focusing on DSA and System Design. But the last six months have been the real turning point for me. I became more consistent, understood DSA patterns better, and spent solid time improving my LLD skills.
I also kept everything organized in my GitHub repositories — every question, every design note — which made revision a lot easier.
I’ve mainly been preparing with banking and fintech roles in mind, since that’s where strong engineering really matters.
Application story
My journey with Card91 started when a LinkedIn connection referred me internally. After that, things moved quickly — the team reached out promptly, scheduled the rounds without much delay, and communicated the results after each stage efficiently.
I went through three rounds in total: a DSA round, an LLD/System Design round, and a final discussion with the hiring manager. The whole process felt smooth, fair, and well-organized.
Why selected/rejected for the role?
I was selected because my fundamentals were strong, and I could clearly explain how I approached each problem. In the DSA round, I stayed calm and talked through my logic. In the LLD round, I kept the design simple, practical, and easy to understand.
Preparation
Duration: 8 months
Topics: Arrays, Strings, Linked Lists, Trees and Graphs, Dynamic Programming, Sliding Window, Binary Search, Hashing, Heaps, a lot of LLD basics, HLD fundamentals, caching, database design, and overall scalability concepts
Tip 1 (DSA): Stick to a fixed set of patterns and solve problems regularly; even 3–4 quality problems a day build solid intuition.
Tip 2 (System Design): Try to understand how real systems work and why certain choices are made, instead of just memorising designs.
Tip 3: Keep your own notes or a GitHub repository — it makes revising much easier and keeps you on track.
Application process
Where: Referral
Eligibility: 1+ YOE in Java. (Salary Package: 9 LPA)
Tip 1: Keep your resume clean and focused on what genuinely represents you.
Tip 2: Highlight the results of your work rather than just listing your responsibilities.
Interview rounds
01
Round
Medium
Video Call
Duration60 minutes
Interview date18 Jan 2025
Coding problem2
The DSA round was around 3 PM, and I was a bit nervous at first because the interviewer seemed very experienced. The environment was calm, though, and once we got into the problem, I settled in. The interviewer was patient and gave me space to think. Overall, it was a straightforward round, aside from the usual interview nerves.
1. Longest Substring Without Repeating Characters
Moderate
30m average time
65% success
0/80
Asked in companies
Given a string input of length n, find the length of the longest substring without repeating characters i.e return a substring that does not have any repeating characters.
Substring is the continuous sub-part of the string formed by removing zero or more characters from both ends.
Problem approach
Step 1: I started with the brute-force approach just to frame the problem, knowing it wouldn’t scale.
Step 2: The interviewer gave a small hint to think in terms of a “window,” which guided me toward the sliding window approach.
Step 3: Since this was a fairly standard question, I quickly moved to the optimal sliding window + HashMap solution. I explained how shifting the start pointer based on last-seen characters reduces the complexity to O(n).
We walked through one example, and the interviewer was satisfied.
2. Robot Return to Origin
Easy
0/40
A robot starts at the origin, position (0, 0), on a 2D Cartesian plane. You are given a string of moves representing a sequence of its movements.
The valid moves are:
'U' (Up): The robot's y-coordinate increases by 1.
'D' (Down): The robot's y-coordinate decreases by 1.
'R' (Right): The robot's x-coordinate increases by 1.
'L' (Left): The robot's x-coordinate decreases by 1.
Your task is to determine if the robot ends up back at the origin (0, 0) after completing all the moves in the sequence.
Problem approach
Step 1: I kept two counters for horizontal and vertical movements.
Step 2: For each move, I updated the counters accordingly.
Step 3: At the end, I checked if both counters were zero.
If they were, the robot had returned to the origin.
02
Round
Medium
Video Call
Duration60 minutes
Interview date22 Jan 2025
Coding problem3
The System Design round was in the afternoon, and I was a bit nervous since it was my first proper one. They started with a few basic questions and then moved into an ATM-style LLD problem, which helped me settle in. The interviewer was friendly and provided small hints whenever I slowed down. After the design part, the discussion shifted to core Java concepts — topics like the Collections framework internals, how the JVM handles memory, a few Spring Boot annotations, and some basics of Spring caching. It felt like a mix of design and core engineering fundamentals, but overall, the round was manageable.
1. System Design
I was asked to design a basic ATM system (LLD style). The interviewer wanted me to define the main entities, describe how they relate to each other, and outline a few basic REST APIs to support core functionalities such as card validation, balance checks, and cash withdrawals.
Problem approach
Tip 1: Start by identifying the key entities: ATM, Card, Account, User, Transaction, and BankServer, and show how they are connected.
Tip 2: Walk through the main flow step by step so the interviewer can follow your thought process.
Tip 3: Study RDBMS thoroughly; knowing how to structure tables and define relationships helps a lot in LLD discussions.
2. DBMS
I was asked a simple SQL question about fetching the second-highest salary from an employee table.
The interviewer also asked how I would handle duplicates, how ORDER BY works internally, and the difference between using LIMIT and a subquery.
Problem approach
Tip 1: Know multiple ways to solve the same SQL question (subquery, LIMIT, ROW_NUMBER()). Interviewers often ask for alternatives.
Tip 2: Understand how sorting and indexing affect performance.
Tip 3: Be comfortable with basic RDBMS concepts such as primary keys, unique constraints, and how duplicates are handled.
3. Java Essentials
I was asked a bunch of short, direct questions from core Java and Spring Boot, such as:
- How does HashMap work internally? (Learn)
- What is the difference between ArrayList and LinkedList? (Learn)
- How is JVM memory structured? (Learn)
- What do common Spring Boot annotations actually do under the hood?
They also touched on the basics of Spring Caching and how the framework manages bean creation.
Problem approach
Tip 1: Have a solid grasp of Java fundamentals, collections internals, the memory model, and common OOP principles.
Tip 2: Know the purpose of key Spring annotations (@Component, @Service, @Repository, @Autowired, @RestController) and how Spring manages beans.
Tip 3: Brush up on how Spring Boot auto-configuration works and the basics of Spring Caching.
03
Round
Easy
HR Round
Duration20 minutes
Interview date27 Jan 2025
Coding problem0
The HR round was in the afternoon and felt straightforward. They asked about my background, why I’m looking for a change, and a few questions about how I work in a team. Toward the end, we discussed compensation expectations and timelines. The HR was friendly and approachable, making the whole conversation feel natural and stress-free.
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