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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.

🏁 Start Here

• How to Learn LLD
• 15 Must-Know OOP Concepts

🧱 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

🗂️ 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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