Introduction to Low-Level Design (LLD) for Beginners

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Best Practices for Effective LLD

• Keep It Modular: Break into small, testable units.
• Prioritize Readability: Use meaningful names, comments.
• Handle Errors Gracefully: Plan for failures.
• Optimize for Performance: Choose efficient data structures.
• Document Thoroughly: Include diagrams, explanations.

Common pitfalls: Over-designing (KISS principle—Keep It Simple, Stupid) or ignoring scalability.

Real-World Examples

Let’s apply LLD to a simple vending machine:

• Classes: VendingMachine, Product, Payment.
• Relationships: VendingMachine has Products.
• Methods: selectProduct(), processPayment().

Extend this to complex systems like e-commerce.

(Alt text: UML Class Diagram for Vending Machine – showing classes and associations.)

Preparing for LLD Interviews: 30 Real Questions with In-Depth Answers

LLD is a staple in FAANG interviews. Here are 30 questions actually asked, with detailed approaches, classes, and key insights. These draw from experiences at companies like Amazon and Google.

1. Design a Parking Lot System Requirements: Multi-level lot, vehicle types (car, bike), entry/exit points, fees. Classes: ParkingLot, Level, Spot, Vehicle, Ticket. Key: Use Factory for vehicles, Strategy for fees. UML: Class diagram with composition. In-depth: Handle spot allocation with priority queue for efficiency; edge cases like full lot use Observer for notifications. Code snippet: class ParkingSpot { boolean isAvailable(); void park(Vehicle v); } Asked at Amazon.
2. Design an Elevator System Requirements: Multiple elevators, floors, requests. Classes: Elevator, Floor, Request, Controller. Key: State pattern for elevator states (moving, idle). In-depth: Optimize with scan algorithm; use queues for requests. Asked at Microsoft.
3. Design a Vending Machine Requirements: Products, payments, inventory. Classes: VendingMachine, Item, Inventory, PaymentProcessor. Key: Singleton for machine instance. In-depth: Handle change with greedy algorithm; integrate Observer for low stock alerts.
4. Design a Chess Game Requirements: Board, pieces, moves, rules. Classes: Board, Piece (abstract), Player. Key: Composite for pieces. In-depth: Validate moves with polymorphism; add undo via Command pattern. Asked at Google.
5. Design Snake and Ladder Game Requirements: Board, players, dice, snakes/ladders. Classes: Game, Board, Player, Dice, SpecialEntity (Snake/Ladder). Key: Strategy for dice rolls. In-depth: Use linked list for board positions; handle wins with state checks. Full code available in resources like GeeksforGeeks.
6. Design Tic-Tac-Toe Requirements: 3×3 board, two players, win detection. Classes: Board, Player, Game. Key: Simple array for board. In-depth: Check wins with row/column/diagonal scans; add AI with minimax.
7. Design BookMyShow (Movie Booking) Requirements: Theaters, shows, seats, bookings. Classes: Theater, Show, Seat, Booking, User. Key: Locking for concurrent bookings. In-depth: Use database transactions; Observer for notifications.
8. Design an ATM Machine Requirements: Accounts, transactions, PIN. Classes: ATM, Account, Transaction. Key: State for sessions. In-depth: Encrypt PIN; handle withdrawals with balance checks.
9. Design a Car Rental System Requirements: Vehicles, users, reservations. Classes: Vehicle, User, Reservation. Key: Factory for vehicle types. In-depth: Availability with calendar integration.
10. Design a Restaurant Management System Requirements: Menus, orders, tables. Classes: Restaurant, Menu, Order, Table. Key: Composite for menus. In-depth: Bill calculation with Strategy.
11. Design a Library Management System Requirements: Books, members, loans. Classes: Library, Book, Member, Loan. Key: Observer for due dates. In-depth: Fine calculation algorithms.
12. Design a Traffic Light System Requirements: Lights, timers, intersections. Classes: TrafficLight, Intersection, Timer. Key: State pattern for colors. In-depth: Synchronize multiple lights.
13. Design Stack Overflow (Q&A Site) Requirements: Questions, answers, votes. Classes: Question, Answer, User, Vote. Key: Composite for threads. In-depth: Ranking with algorithms.
14. Design a URL Shortener (LLD Focus) Requirements: Shorten, redirect. Classes: URLService, Database. Key: Hashing for shorts. In-depth: Collision handling.
15. Design a File System Requirements: Directories, files, permissions. Classes: FileSystem, Directory, File. Key: Composite pattern. In-depth: Tree traversal.
16. Design a Logger System Requirements: Levels, appenders. Classes: Logger, Appender. Key: Chain of Responsibility. In-depth: Async logging.
17. Design a Cache System Requirements: Eviction, hits. Classes: Cache, Entry. Key: LRU with doubly linked list. In-depth: Thread-safety.
18. Design a Pub-Sub System Requirements: Topics, subscribers. Classes: Publisher, Subscriber, Topic. Key: Observer pattern. In-depth: Scalable with queues.
19. Design a Rate Limiter Requirements: Requests per time. Classes: RateLimiter, Bucket. Key: Token bucket algorithm. In-depth: Distributed versions.
20. Design a Leaderboard Requirements: Scores, rankings. Classes: Leaderboard, PlayerScore. Key: Sorted set. In-depth: Real-time updates.
21. Design an Online Judge Requirements: Submissions, judging. Classes: Judge, Submission, TestCase. Key: Queue for jobs. In-depth: Sandbox execution.
22. Design a Meeting Scheduler Requirements: Calendars, conflicts. Classes: Scheduler, Meeting, Calendar. Key: Interval trees for checks. In-depth: Notifications.
23. Design Ride-Sharing (Uber LLD) Requirements: Riders, drivers, trips. Classes: User, Driver, Trip. Key: State for trip status. In-depth: Matching algorithms.
24. Design a Social Network Feed Requirements: Posts, feeds. Classes: Post, User, Feed. Key: Fan-out for distribution. In-depth: Pagination.
25. Design a Notification System Requirements: Channels, users. Classes: Notification, Channel. Key: Strategy for types. In-depth: Batching.
26. Design Inventory Management Requirements: Products, stock. Classes: Inventory, Product. Key: Observer for low stock. In-depth: Transactions.
27. Design an E-commerce Cart Requirements: Items, checkout. Classes: Cart, Item. Key: Session-based. In-depth: Discounts with Strategy.
28. Design a Payment System Requirements: Gateways, transactions. Classes: PaymentGateway, Transaction. Key: Adapter for providers. In-depth: Security with encryption.
29. Design a Bank System Requirements: Accounts, transfers. Classes: Bank, Account. Key: Locking for concurrency. In-depth: Audit logs.
30. Design a Hospital Management System Requirements: Patients, doctors, appointments. Classes: Hospital, Patient, Doctor, Appointment. Key: Scheduler integration. In-depth: Privacy compliance.

These questions emphasize practical application—focus on requirements, diagrams, and code during prep.

Resources and Next Steps

• Books: “Design Patterns” by Gang of Four.
• Online: Educative.io, GeeksforGeeks.
• Practice: LeetCode discussions, GitHub repos.

Ready to practice? Enroll in our courses for hands-on projects. What’s your first LLD project? Share in the comments!

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