Low-Level Design — Interview Prep

A collection of classic LLD problems solved in Python, scoped for a 45-minute interview round. Each folder contains a design.py with the implementation and a Readme.md with the problem description, scope, and patterns used.

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Problems

#	Problem	Patterns Used
1	Car Rental System	Strategy, Factory, Observer, Singleton
2	Parking Lot	Strategy (fee + payment), Factory, Composition
3	Chess Game	Template Method, Factory, Singleton
4	Elevator	Strategy (scheduling), Observer, Command
5	Snake and Food Game	Template Method, Strategy, Factory
6	Tic Tac Toe Game	Strategy, Factory, Observer
7	File System	Composite, Singleton
8	Rate Limiter	Strategy, Factory
9	Splitwise	Strategy, Factory, Observer
10	Resource Lease System	FIFO Deque, Lazy Deletion, Thread Safety
11	Movie Booking System	Strategy, Abstract Provider, Thread Safety

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Design Patterns

#	Pattern	Folder
1	Singleton Pattern	Explanation + code examples

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Common Design Patterns Across Problems

Pattern	Where It Appears
Strategy	Payment methods (Car Rental, Parking Lot), player input (Snake, Tic Tac Toe), elevator scheduling, rate limiter algorithms, expense splitting (Splitwise)
Factory	Vehicle creation (Car Rental, Parking Lot), piece creation (Chess), player creation (Snake, Tic Tac Toe), rate limiter creation
Observer	Booking notifications (Car Rental), floor display (Elevator), game state changes (Tic Tac Toe), balance updates (Splitwise)
Singleton	System entry point (Car Rental), Board (Chess), FileSystem
Template Method	Piece movement validation (Chess), snake move handling (Snake)
Composite	File/Directory hierarchy (File System)
Command	Elevator requests (Elevator)
FIFO Deque + Lazy Deletion	O(1) amortised token expiry (Resource Lease System)
Threading Lock	Thread-safe token grant / release (Resource Lease System, Rate Limiter, Movie Booking System)
Provider (Abstract Factory variant)	Pluggable lock backend (Movie Booking System)

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How to Use

1. Pick a problem and read its Readme.md for scope and interview strategy.
2. Open design.py — code is ordered top-down (enums → models → services → orchestrator).
3. Look for # TODO: comments — these mark features intentionally left out of scope for a 45-min round but worth mentioning verbally to the interviewer.
4. Practice explaining class relationships and pattern choices out loud — that's what interviewers evaluate.