Interview prep
A shop ledger page, 1899. Every line adds up.
LLD / Java / OOP Interview Questions Bank
Part A: 60 conceptual questions with crisp answers. Part B: 30 LLD problems with difficulty and patterns.
12 min read
LLD Series: Hotel Booking System - LLD
Design a hotel booking system: search availability for a date range, book rooms, cancel, and make sure the hotel is never overbooked even with concurrent requests. Support different pricing (weekend,...
11 min read
LLD Series: In-Memory Pub/Sub (Kafka-lite) - LLD
Design an in-memory publish-subscribe system. Publishers publish messages to topics; subscribers subscribe to topics and receive messages. Delivery should be asynchronous, a slow subscriber must not...
10 min read
LLD Series: Logger Framework (Low-Level Design)
Asked at: Visa, Mastercard, Amex, PayU (classic machine-coding warm-up; checks patterns knowledge). Interviewers expect: Singleton, Chain of Responsibility, Strategy for formatter/appender, and a...
12 min read
LLD Series: Notification Service (Low-Level Design)
Asked at: PayU, Amex, Visa and Mastercard (OTP / transaction alerts / fraud alerts), NPCI-ecosystem apps (UPI collect and debit alerts). Interviewers watch for: Strategy per channel, Observer for...
13 min read
LLD Series: BookMyShow (Movie Ticket Booking) (LLD)
Design a movie ticket booking system. Users browse shows, pick seats, pay and get a ticket. Two users must never book the same seat. The real question is concurrency: temporary seat holds with...
15 min read
LLD Series: Elevator System (LLD)
Design an elevator system for a building with N floors and M elevators. Users press hall buttons (floor + direction) and cabin buttons (destination). Decide which elevator serves a request, and how...
12 min read
LLD Series: LRU / LFU Cache (Low-Level Design)
Phrasings: LeetCode 146/460 but as a class design, Design an in-memory cache like Guava/Caffeine, cache BIN/merchant/config lookups. Asked heavily at Visa, Mastercard, NPCI, Amex, PayU...
15 min read
LLD Series: Rate Limiter (Low-Level Design)
Typical phrasing: Design an API throttler, Limit 100 req/min per user, Implement token bucket. Asked at Visa/Mastercard (API gateway protection, card-network throttling), NPCI (UPI PSP throttling per...
13 min read
LLD Series: Payment Gateway with Routing, Retries and Webhooks (Low-Level Design)
Design a payment gateway (Razorpay / PayU / Stripe style). Merchants submit payments by card, UPI, net-banking or wallet. The gateway routes to a bank/acquirer, falls back if one is down, retries...
29 min read
DSA Series: Tree, BST and Trie (Blind 75, Java)
14 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. Recursion on trees, BFS levels, BST invariants, serialization, tries.
39 min read
LLD Series: Vending Machine (Low-Level Design)
Design a vending machine. A user inserts coins/notes, selects a product, the machine dispenses it and returns change. Handle sold-out items, insufficient money, cancellation and refunds.
16 min read
DSA Series: String (Blind 75, Java)
10 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. Sliding window, hashing by signature, stack matching, palindromes, encoding.
25 min read
LLD Series: Digital Wallet (Low-Level Design)
Design a digital wallet (Paytm / PhonePe / PayPal style). Users can add money, transfer to other users, and view history. Make transfers safe under concurrent requests, idempotent on retries, and...
21 min read
DSA Series: Linked List (Blind 75, Java)
6 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. Dummy node, fast/slow pointers, reversal, merging sorted lists.
17 min read
LLD Series: Splitwise / Expense Sharing (Low-Level Design)
Design Splitwise. Users add expenses and split them equally, by exact amounts, or by percentage. Show who owes whom. Add a 'simplify debts' feature that minimises the number of transactions.
18 min read
DSA Series: Heap (Blind 75, Java)
2 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. PriorityQueue patterns: top-K frequency and the two-heap running median.
10 min read
LLD Series: ATM (Low-Level Design)
Design an ATM. A user inserts a card, enters a PIN, and can withdraw cash, deposit or check balance. The machine must dispense the right notes. Model the machine's behaviour so that invalid...
19 min read
DSA Series: Interval (Blind 75, Java)
5 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. Sort-and-sweep, greedy by end time, min-heap for rooms.
15 min read
LLD Series: Parking Lot (Low-Level Design)
Design a multi-floor parking lot. Vehicles enter, get a ticket and a spot, and pay on exit. Support different vehicle types and pricing. Write working code in 60 minutes.
15 min read
DSA Series: Graph (Blind 75, Java)
8 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. BFS, DFS, topological sort and union-find with the Blind 75 graph problems.
28 min read
LLD Interview Framework (45-60 minute round)
Goal: show structured thinking, clean OO modeling, working code, and awareness of extensibility/concurrency. Interviewers rarely want a perfect system; they want a runnable core with sound design and...
8 min read
DSA Series: Matrix (Blind 75, Java)
4 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. In-place marking, boundary peeling, transpose-reverse rotation, grid backtracking.
21 min read
LLD Series: Java Concurrency Essentials for LLD Rounds
In fintech LLD rounds the follow-up is almost always: now make it thread-safe (two threads debit the same wallet, two users book the same seat, two requests carry the same idempotency key). Know the...
16 min read
DSA Series: Dynamic Programming (Blind 75, Java)
11 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. State, transition, base case, memoization vs tabulation, space optimisation.
53 min read
LLD Series: Design Patterns That Actually Come Up in LLD Rounds
Rule: name the pattern only after you have stated the problem it solves. Payment methods vary and I don't want if/else on type, so Strategy beats I'll use Strategy.
19 min read
DSA Series: Binary / Bit Manipulation (Blind 75, Java)
5 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. XOR, carry arithmetic, Kernighan bit trick, DP over bits.
15 min read
LLD Series: OOP and SOLID (Payments Flavored)
All snippets are Java 11, compiled and run. Bad versions compile too; they are bad design, not bad syntax.
12 min read
DSA Series: Array (Blind 75, Java)
10 Blind 75 problems with brute force to optimal approaches, tested Java code, time/space complexity, how interviewers ask and follow-ups. Hash maps, prefix/suffix products, Kadane, binary search on rotated arrays, two pointers.
28 min read
HLD Fundamentals for Beginners: Estimation, Building Blocks and a Payments Lens
A beginner-friendly system design primer: the 6-step framework, back-of-envelope QPS math, L4 vs L7 load balancing, caching, databases, queues, reliability patterns and how a payment flows end to end.
7 min read
DSA Patterns Roadmap for Beginners: 9 Patterns With Runnable Java
Learn DSA as a small set of patterns instead of hundreds of problems. Nine patterns, each with a trigger phrase, a Java example you can run, complexity, and where it shows up in real design problems.
8 min read
LLD Series: DSA and Java Basics for LLD Interviews: A Gentle Primer for Beginners
If your DSA base feels shaky, read this first. It explains just the data structures, Big-O, concurrency and money rules that the LLD series actually uses, with runnable Java.
8 min read
LLD Series: Interview Prep Series: Start Here (DSA, HLD and LLD for Visa, Mastercard, NPCI, Amex, PayU)
Index and study plan for a DSA, HLD and LLD interview-prep series in Java for product and fintech companies, with a progress tracker and a searchable glossary for every abbreviation.
4 min read