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Design Problems - Low Level Design

Course Contents

Getting Ready: Parking Lot System

Requirements for the Parking Lot

Use Case Diagram

Activity Diagram

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Tic Tac Toe

Requirements for Tic Tac Toe

Code Implementation

Extending to NxN Board

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Snake and Ladder

Requirements for Snake and Ladder

Activity Diagram

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: LRU Cache

Requirements for the LRU Cache

Code Implementation — HashMap + Doubly Linked List

Thread-Safe LRU Cache

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Logging Framework

Requirements for the Logging Framework

Code Implementation

Adding New Log Sinks (Extensibility)

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Vending Machine

Requirements for the Vending Machine

State Machine Diagram

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Extension: Multiple Payment Methods

Summary & Key Takeaways

Getting Ready: Task Management System

Requirements for the Task Manager

Code Implementation

Undo/Redo with Command Pattern

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Elevator System

Requirements for the Elevator System

Use Case Diagram

State Machine Diagram

Scheduling Algorithms (FCFS, SCAN, LOOK)

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Library Management System

Requirements for the Library System

Use Case Diagram

Sequence Diagram — Book Checkout Flow

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Meeting Scheduler

Requirements for the Meeting Scheduler

Use Case Diagram

Sequence Diagram — Schedule a Meeting

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: ATM System

Requirements for the ATM

State Machine Diagram

Sequence Diagram — Withdraw Cash

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Car Rental System

Requirements for the Car Rental System

Use Case Diagram

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Extension: Insurance & Add-Ons

Summary & Key Takeaways

Getting Ready: Pub-Sub System

Requirements for the Pub-Sub System

Sequence Diagram — Publish & Subscribe Flow

Code Implementation

Thread-Safe Message Delivery

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Hotel Management System

Requirements for the Hotel System

Use Case Diagram

State Machine Diagram — Room Status

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Splitwise

Requirements for Splitwise

Sequence Diagram — Split an Expense

Code Implementation

Debt Simplification Algorithm

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Digital Wallet

Requirements for the Digital Wallet

Sequence Diagram — Transfer Money

Code Implementation

Transaction Safety & Consistency

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Traffic Signal System

Requirements for the Traffic Signal

State Machine Diagram

Code Implementation

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Stack Overflow

Requirements for Stack Overflow

Use Case Diagram

Code Implementation

Reputation & Voting System

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Movie Ticket Booking

Requirements for the Booking System

Use Case Diagram

Sequence Diagram — Book a Ticket

State Machine Diagram — Booking Status

Code Implementation

Concurrent Seat Locking

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Online Shopping

Requirements for the Shopping System

Use Case Diagram

State Machine Diagram — Order Lifecycle

Sequence Diagram — Place an Order

Code Implementation

Design Patterns Applied

Payment Strategy & Extensibility

Summary & Key Takeaways

Getting Ready: Chess Game

Requirements for the Chess Game

Class Diagram — Piece Hierarchy

Class Diagram — Board & Game

Sequence Diagram — Make a Move

Code Implementation — Board & Pieces

Code Implementation — Game Logic & Rules

Check, Checkmate & Stalemate Detection

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Ride-Sharing Service

Requirements for the Ride-Sharing System

Use Case Diagram

State Machine Diagram — Ride Lifecycle

Sequence Diagram — Request a Ride

Code Implementation

Matching & Surge Pricing Algorithms

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Music Streaming

Requirements for the Streaming Service

Sequence Diagram — Play a Song

Code Implementation

Playlist & Queue Management

Subscription Tiers with Decorator

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Social Network

Requirements for the Social Network

Use Case Diagram

Sequence Diagram — Create a Post

Code Implementation — Users & Friendships

Code Implementation — Posts & Feed

News Feed Generation Algorithm

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Food Delivery Service

Requirements for the Food Delivery System

Use Case Diagram

State Machine Diagram — Order Lifecycle

Sequence Diagram — Place an Order

Code Implementation

Delivery Assignment Strategy

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Stock Brokerage

Requirements for the Stock Brokerage

Use Case Diagram

Sequence Diagram — Place an Order

Code Implementation — Order Book

Code Implementation — Matching Engine

Price Alerts with Observer

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Online Auction

Requirements for the Auction System

State Machine Diagram — Auction Lifecycle

Sequence Diagram — Place a Bid

Code Implementation

Auction Types (English, Dutch, Sealed-Bid)

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Airline Management

Requirements for the Airline System

Use Case Diagram

Sequence Diagram — Book a Flight

Code Implementation

Multi-Class Pricing Strategy

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Restaurant Management

Requirements for the Restaurant System

State Machine Diagram — Table & Order Status

Sequence Diagram — Place an Order

Code Implementation

Kitchen Display with Observer

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Course Registration

Requirements for the Course Registration System

Sequence Diagram — Register for a Course

Code Implementation

Prerequisite Validation (DAG)

Waitlist with Observer

Design Patterns Applied

Summary & Key Takeaways

Getting Ready: Notification System

Requirements for the Notification System

Use Case Diagram

Sequence Diagram — Send Notification Flow

Code Implementation — Multi-Channel (Email, SMS, Push)

Priority Queue & Rate Limiting

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: File Storage System

Requirements for the File Storage System

Use Case Diagram

Sequence Diagram — Upload & Download Flow

Code Implementation — Files, Folders, Permissions

Chunked Uploads & Deduplication

Design Patterns Applied

Testing & Edge Cases

Summary & Key Takeaways

Getting Ready: Blocking Queue

Requirements & Concurrency Challenges

Code Implementation — Python

Code Implementation — Java

Testing Concurrent Behavior

Summary & Key Takeaways

Getting Ready: Thread Pool

Requirements & Design Decisions

Code Implementation — Python

Code Implementation — Java

Graceful Shutdown & Edge Cases

Summary & Key Takeaways

Getting Ready: Rate Limiter

Requirements for the Rate Limiter

Token Bucket Algorithm

Sliding Window Algorithm

Leaky Bucket Algorithm

Code Implementation

Summary & Key Takeaways

Getting Ready: Concurrent HashMap

Requirements & Concurrency Challenges

Code Implementation — Segment Locking

Code Implementation — CAS Operations

Performance Benchmarking

Summary & Key Takeaways

Top 10 Tips for LLD Interviews

Common Mistakes and How to Avoid Them

Practice Plan — 30, 60, 90 Day Roadmaps

Course Conclusion & Next Steps

Prerequisite Validation (DAG)

Model course prerequisites as a Directed Acyclic Graph, detect cycles with three-color DFS, validate full prerequisite chains, and generate valid course orderings with topological sort.

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