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This repository contains resources to learn Low Level Design (LLD) / Object Oriented Design (OOD) and prepare for interviews. It covers OOP fundamentals, design patterns, UML, concurrency and commonly asked interview questions.

👉 For a better and more comprehensive experience, checkout the LLD page at AlgoMaster.io

🧱 OOP Fundamentals

• Classes and Objects
• Enums
• Interfaces
• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

🔗 Class Relationships

• Association
• Aggregation
• Composition
• Dependency

🧭 Design Principles

• DRY Principle
• YAGNI Principle
• KISS Principle
• SOLID Principles with Pictures
• SOLID Principles with Code

🧩 Design Patterns

Creational Patterns	Structural Patterns	Behavioral Patterns
Singleton	Adapter	Iterator
Factory Method	Bridge	Observer
Abstract Factory	Composite	Strategy
Builder	Decorator	Command
Prototype	Facade	State
	Flyweight	Template Method
	Proxy	Visitor
		Mediator
		Memento
		Chain of Responsibility

🗂️ UML

• Class Diagram
• Use Case Diagram
• Sequence Diagram
• Activity Diagram
• State Machine Diagram

⏱️ Concurrency and Multi-threading Concepts

Concurrency 101

• Introduction to Concurrency
• Concurrency vs Parallelism
• Processes vs Threads
• Thread Lifecycle and States
• Race Conditions and Critical Sections

Synchronization Primitives

• Mutex (Mutual Exclusion)
• Semaphores
• Condition Variables
• Coarse-grained vs Fine-grained Locking
• Reentrant Locks
• Try-Lock and Timed Locking
• Compare-and-Swap (CAS)

Concurrency Challenges

• Deadlock
• Livelock

Concurrency Patterns

• Signaling Pattern
• Thread Pool Pattern
• Producer-Consumer Pattern
• Reader-Writer Pattern

✅ How to Answer a LLD Interview Problem

💻 Low Level Design Interview Problems

Easy Problems

• Design Parking Lot
• Design Stack Overflow
• Design a Vending Machine
• Design Logging Framework
• Design Traffic Signal Control System
• Design Coffee Vending Machine
• Design a Task Management System

Medium Problems

• Design ATM
• Design LinkedIn
• Design LRU Cache
• Design Tic Tac Toe Game
• Design Pub Sub System
• Design an Elevator System
• Design Car Rental System
• Design an Online Auction System
• Design Hotel Management System
• Design a Digital Wallet Service
• Design Airline Management System
• Design a Library Management System
• Design a Social Network like Facebook
• Design Restaurant Management System
• Design a Concert Ticket Booking System

Hard Problems

• Design CricInfo
• Design Splitwise
• Design Chess Game
• Design a Snake and Ladder game
• Design Ride-Sharing Service like Uber
• Design Course Registration System
• Design Movie Ticket Booking System
• Design Online Shopping System like Amazon
• Design Online Stock Brokerage System
• Design Music Streaming Service like Spotify
• Design Online Food Delivery Service like Swiggy

⏱️ Concurrency and Multi-threading Problems

• Print FooBar Alternately
• Print Zero Even Odd
• Fizz Buzz Multithreaded
• Building H2O Molecule
• Design Thread-Safe Cache with TTL
• Design Concurrent HashMap
• Design Thread-Safe Blocking Queue
• Design Concurrent Bloom Filter
• Multi-threaded Merge Sort

📇 Courses

• Master LLD Interviews - AlgoMaster.io
• Master Concurrency Interviews - AlgoMaster.io

📚 Books

• Head First Design Patterns
• Clean Code
• Refactoring: Improving the Design of Existing Code

📩 Newsletter

• AlgoMaster Newsletter

Additional resources

• Coursera - Object-Oriented Design
• Coursera - Design Patterns
• Github - Awesome Design Patterns

🤝 Contributing

Contributions are welcome! If you'd like to add a new problem, improve existing content, or fix errors:

1. Fork the repository
2. Create a feature branch: git checkout -b feature/your-feature-name
3. Commit your changes: git commit -m 'Add some feature'
4. Push to the branch: git push origin feature/your-feature-name
5. Submit a pull request

Please make sure to update Readme files and documentation as appropriate.

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