Low-Level Design (LLD)
Object-oriented design, machine coding, and API design
| # | Topic | Importance | Status | Key Concepts | Best Resources |
|---|---|---|---|---|---|
| 1 | LRU Cache | critical | HashMap + DLL, O(1) get/put, eviction, thread-safe variant | ||
| 2 | SOLID Principles | critical | SRP, OCP, LSP, ISP, DIP — with real-world code examples | ||
| 3 | Design Patterns (GoF) | critical | All 23 Gang of Four patterns — creational, structural, behavioural | ||
| 4 | Parking Lot System | high | Singleton, Strategy (pricing), multiple vehicle types, tickets, floors | ||
| 5 | Elevator System | high | SCAN algorithm, State machine (IDLE/UP/DOWN), request scheduling | ||
| 6 | ATM System | high | State machine (Idle→Card→Pin→Amount→Dispense), card, account, transactions | ||
| 7 | Vending Machine | high | State pattern, inventory management, payment processing, change dispensing | ||
| 8 | Chess / Board Game | high | Polymorphic pieces, move validation, check/checkmate detection, game state | ||
| 9 | Splitwise | high | Group expenses, equal/exact/percentage splits, debt simplification algorithm | ||
| 10 | Movie Ticket Booking | high | Theatre/show/seat hierarchy, concurrent seat locking, payment, optimistic locking | ||
| 11 | Hotel Booking | high | Room availability, date range queries, concurrent booking, strategy pricing | ||
| 12 | Food Delivery (Swiggy/Zomato) | high | Restaurant, menu, cart, order lifecycle, delivery agent assignment | ||
| 13 | Observer Pattern | critical | Subject notifies observers, event bus, decoupled pub-sub in-process | ||
| 14 | Strategy Pattern | critical | Swap algorithms at runtime, payment strategies, sorting strategies | ||
| 15 | State Pattern | critical | Object behaviour changes with state, FSM, vending machine, ATM, order lifecycle | ||
| 16 | Concurrency & Thread Safety | high | Race conditions, deadlocks, mutex, semaphore, ReentrantLock, ThreadLocal | ||
| 17 | Library Management System | medium | Books, members, lending, fines, reservations, search by author/ISBN | ||
| 18 | Cricinfo / Live Score | medium | Match, innings, over, players, live updates, scorecard, commentary | ||
| 19 | Truecaller | medium | Trie/Prefix tree for contact search, Bloom filter for spam detection | ||
| 20 | Snake and Ladder Game | medium | Board, Dice, Player, Game entities, position updates | ||
| 21 | Amazon Locker Service | high | Locker allocation, package sizing, state pattern for locker status | ||
| 22 | Car Rental (Zoomcar) | high | Vehicle inventory, booking schedules, payment processing | ||
| 23 | Distributed ID Generator | high | Snowflake ID, timestamp + worker ID + sequence number | ||
| 24 | In-Memory Logger | medium | Log levels, appenders, thread-safe message queueing | ||
| 25 | Cab Booking (Uber/Ola) | high | Driver matching, trip lifecycle, location tracking, pricing strategy | ||
| 26 | Tic Tac Toe | medium | Board state, win conditions checking, player turns | ||
| 27 | Configuration Management | medium | Pub-sub for config updates, hierarchical config structures | ||
| 28 | JSON Parser | high | Lexical analysis, tokenization, AST generation, recursive descent parsing | ||
| 29 | e-Commerce Review System | medium | Ratings, reviews, moderation, helpful votes | ||
| 30 | In-memory Data Streaming | medium | Data streaming, Pub-Sub, message queues | ||
| 31 | In-Memory DB with Indexing | high | Data structures for fast retrieval, indexing mechanisms | ||
| 32 | Online Book Reader System | medium | Book management, user sessions, reading progress | ||
| 33 | Logistics System | medium | Tracking shipments, delivery routes, vehicle assignment | ||
| 34 | Maps Navigator Client | medium | Different transportation types, routing strategies | ||
| 35 | TikTok | high | Video feed, recommendation engine, real-time engagement | ||
| 36 | Pinterest | high | Image pinning, boards, feed generation |