Object Oriented Programming

Low Level Design Interviews

A free, 31-module low-level design interview course - nine methodology modules covering the 45-minute interview end-to-end, followed by 22 worked case studies from parking lots to ML inference pipelines.

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Low Level Design Interviews

This is the course I wish I'd had before my eleventh LLD rejection. Two halves: a nine-module methodology stack that breaks the 45-minute LLD interview into the moves senior engineers actually make under pressure, and 22 case studies that take one concrete problem each from "the interviewer just asked it" through requirements, modeling, classes, code, edge cases, and trade-offs.

Part One — Methodology (Modules 1-9). What an LLD interview actually is and what interviewers listen for. How to clarify the problem in 5-6 minutes without drifting. How to model entities, relationships, and invariants before reaching for code. How to design the structure (classes, packages, boundaries). How to implement under time pressure without painting yourself into a corner. Communication as a cross-cutting skill — narrating decisions, surfacing trade-offs, fielding follow-ups. How to verify and walk through your design. How to recover when the interviewer pushes back or you realise mid-flight your model is wrong. And the mock interview methodology that turns deliberate practice into offer signal.

Part Two — Case Studies (Modules 10-31). Twenty-two end-to-end worked examples: Parking Lot, LRU Cache, Multi-Elevator, Rate Limiter, Chat System, Notification Service, Hotel Booking, Task Management, Vending Machine, Mobile System Design (iOS), ML Inference Pipeline, Career Trajectory Framework, Tic-Tac-Toe, Snake and Ladder, Chess Game, Connect Four, Card Game Framework, Simple Calculator, File System Tree, URL Shortener, Movie Ticket Booking, Snake Game. Each case study has the same seven-topic shape: the problem itself, then six numbered methodology steps applied to that specific problem.

What you will learn

• The 45-minute LLD interview anatomy — what each segment is for and how to budget time
• What interviewers actually evaluate — and how to demonstrate the skills explicitly
• A repeatable six-step methodology — requirements → entities → classes → code → edge cases → trade-offs
• Communication patterns — narrating decisions, surfacing trade-offs, recovering from pushback
• Twenty-two end-to-end worked case studies covering the full LLD problem space
• Recovery playbooks — what to do when your model is wrong, time is short, or the interviewer disagrees
• A practice methodology you can apply to any new LLD problem after the course

Prerequisites

• LLD Fundamentals (recommended) — class design, SOLID, design pattern catalogue

Course outline

Module 1: Understanding the LLD Interview

Understanding the LLD Interview — 5 topics.

• What an LLD Interview Actually Is 9 min read
• What Interviewers Actually Listen For 9 min read Pro lesson
• The 45-Minute Anatomy 10 min read Pro lesson
• What Strong Performance Looks Like 9 min read Pro lesson
• What Failure Looks Like - and Recovery 11 min read

Module 2: Clarifying the Problem

Clarifying the Problem — 5 topics.

• Why Clarification is Everything 9 min read Pro lesson
• The Five Question Categories 9 min read Pro lesson
• Making Assumptions Explicit 9 min read Pro lesson
• Reading the Interviewer 9 min read Pro lesson
• The Confirmation Loop 10 min read Pro lesson

Module 3: Modeling the Problem

Modeling the Problem — 5 topics.

• The Modeling Mindset 9 min read Pro lesson
• Identifying Entities 9 min read Pro lesson
• Naming Entities Well 9 min read Pro lesson
• Relationships and Boundaries 9 min read Pro lesson
• The Commit and Iterate Pattern 10 min read Pro lesson

Module 4: Designing the Structure

Designing the Structure — 5 topics.

• From Entities to Classes 10 min read Pro lesson
• Method Signatures and Responsibilities 10 min read Pro lesson
• Composition over Inheritance in Practice 10 min read
• When to Reach for a Pattern 10 min read Pro lesson
• Defending Your Design Choices 9 min read Pro lesson

Module 5: Implementation Under Time Pressure

Implementation Under Time Pressure — 5 topics.

• The Implementation Mindset 10 min read Pro lesson
• What to Implement First 9 min read Pro lesson
• Writing Readable Code Under Pressure 10 min read
• When Code Doesnt Work in Front of You 10 min read Pro lesson
• Knowing When Youre Done 10 min read Pro lesson

Module 6: Communication as a Cross-Cutting Skill

Communication as a Cross-Cutting Skill — 5 topics.

• Thinking Out Loud Productively 9 min read Pro lesson
• The Vocabulary of Design Decisions 10 min read Pro lesson
• Asking Productive Questions Throughout 9 min read Pro lesson
• Articulating Trade Offs in Real Time 9 min read Pro lesson
• The Final Summary 10 min read Pro lesson

Module 7: Verification and Walkthrough

Verification and Walkthrough — 5 topics.

• The Verification Mindset 9 min read Pro lesson
• Walking Through Scenarios 9 min read Pro lesson
• Edge Cases and What to Surface 9 min read Pro lesson
• Handling the Bug You Find 9 min read Pro lesson
• Discussing Extensions and Scale 10 min read Pro lesson

Module 8: Recovery in Real Time

Recovery in Real Time — 5 topics.

• When Your Design Has a Major Flaw 10 min read Pro lesson
• When Youre Behind on Time 9 min read Pro lesson
• When the Interviewer Changes Direction 9 min read Pro lesson
• When You Dont Know Something 10 min read Pro lesson
• When Youre Losing the Interviewer 8 min read Pro lesson

Module 9: Mock Interview Methodology

Mock Interview Methodology — 5 topics.

• Designing a Mock Interview Practice 10 min read Pro lesson
• Self Evaluation After a Mock 10 min read Pro lesson
• Working with a Mock Partner 10 min read Pro lesson
• Tracking Progress Across Mocks 9 min read Pro lesson
• The Final Two Weeks Before a Real Interview 10 min read Pro lesson

Module 10: Case Study: Parking Lot

Case Study: Parking Lot — 7 topics.

• The Problem 13 min read
• Step 1: Requirements 13 min read
• Step 2: Core Entities 17 min read
• Step 3: Classes and Relationships 13 min read
• Step 4: Code Implementation 13 min read
• Step 5: Edge Cases and Tests 13 min read
• Step 6: Extensibility and Trade-offs 15 min read

Module 11: Case Study: LRU Cache

Case Study: LRU Cache — 7 topics.

• The Problem 13 min read Pro lesson
• Step 1: Requirements 13 min read Pro lesson
• Step 2: Core Entities 17 min read Pro lesson
• Step 3: Classes and Relationships 14 min read Pro lesson
• Step 4: Code Implementation 13 min read Pro lesson
• Step 5: Edge Cases and Tests 15 min read Pro lesson
• Step 6: Extensibility and Trade-offs 15 min read Pro lesson

Module 12: Case Study: Multi-Elevator

Case Study: Multi-Elevator — 7 topics.

• The Problem 7 min read Pro lesson
• Step 1: Requirements 8 min read Pro lesson
• Step 2: Core Entities 15 min read Pro lesson
• Step 3: Classes and Relationships 12 min read Pro lesson
• Step 4: Code Implementation 11 min read Pro lesson
• Step 5: Edge Cases and Tests 14 min read Pro lesson
• Step 6: Extensibility and Trade-offs 11 min read Pro lesson

Module 13: Case Study: Rate Limiter

Case Study: Rate Limiter — 7 topics.

• The Problem 12 min read Pro lesson
• Step 1: Requirements 8 min read Pro lesson
• Step 2: Core Entities 15 min read Pro lesson
• Step 3: Classes and Relationships 12 min read Pro lesson
• Step 4: Code Implementation 15 min read Pro lesson
• Step 5: Edge Cases and Tests 15 min read Pro lesson
• Step 6: Extensibility and Trade-offs 14 min read Pro lesson

Module 14: Case Study: Chat System

Case Study: Chat System — 7 topics.

• The Problem 13 min read Pro lesson
• Step 1: Requirements 13 min read Pro lesson
• Step 2: Core Entities 16 min read Pro lesson
• Step 3: Classes and Relationships 15 min read Pro lesson
• Step 4: Code Implementation 11 min read Pro lesson
• Step 5: Edge Cases and Tests 11 min read Pro lesson
• Step 6: Extensibility and Trade-offs 18 min read Pro lesson

Module 15: Case Study: Notification Service

Case Study: Notification Service — 7 topics.

• The Problem 8 min read Pro lesson
• Step 1: Requirements 7 min read Pro lesson
• Step 2: Core Entities 14 min read Pro lesson
• Step 3: Classes and Relationships 11 min read Pro lesson
• Step 4: Code Implementation 11 min read Pro lesson
• Step 5: Edge Cases and Tests 13 min read Pro lesson
• Step 6: Extensibility and Trade-offs 14 min read Pro lesson

Module 16: Case Study: Hotel Booking

Case Study: Hotel Booking — 7 topics.

• The Problem 12 min read Pro lesson
• Step 1: Requirements 17 min read Pro lesson
• Step 2: Core Entities 18 min read Pro lesson
• Step 3: Classes and Relationships 16 min read Pro lesson
• Step 4: Code Implementation 17 min read Pro lesson
• Step 5: Edge Cases and Tests 19 min read Pro lesson
• Step 6: Extensibility and Trade-offs 16 min read Pro lesson

Module 17: Case Study: Task Management

Case Study: Task Management — 7 topics.

• The Problem 4 min read Pro lesson
• Step 1: Requirements 6 min read Pro lesson
• Step 2: Core Entities 12 min read Pro lesson
• Step 3: Classes and Relationships 9 min read Pro lesson
• Step 4: Code Implementation 9 min read Pro lesson
• Step 5: Edge Cases and Tests 11 min read Pro lesson
• Step 6: Extensibility and Trade-offs 10 min read Pro lesson

Module 18: Case Study: Vending Machine

Case Study: Vending Machine — 7 topics.

• The Problem 10 min read Pro lesson
• Step 1: Requirements 12 min read Pro lesson
• Step 2: Core Entities 15 min read Pro lesson
• Step 3: Classes and Relationships 14 min read Pro lesson
• Step 4: Code Implementation 16 min read Pro lesson
• Step 5: Edge Cases and Tests 15 min read Pro lesson
• Step 6: Extensibility and Trade-offs 12 min read Pro lesson

Module 19: Case Study: Mobile System Design (iOS)

Case Study: Mobile System Design (iOS) — 7 topics.

• The Problem 4 min read Pro lesson
• Step 1: Requirements 4 min read Pro lesson
• Step 2: Core Entities 9 min read Pro lesson
• Step 3: Classes and Relationships 10 min read Pro lesson
• Step 4: Code Implementation 9 min read Pro lesson
• Step 5: Edge Cases and Tests 9 min read Pro lesson
• Step 6: Extensibility and Trade-offs 8 min read Pro lesson

Module 20: Case Study: ML Inference Pipeline

Case Study: ML Inference Pipeline — 7 topics.

• The Problem 4 min read Pro lesson
• Step 1: Requirements 4 min read Pro lesson
• Step 2: Core Entities 9 min read Pro lesson
• Step 3: Classes and Relationships 8 min read Pro lesson
• Step 4: Code Implementation 8 min read Pro lesson
• Step 5: Edge Cases and Tests 11 min read Pro lesson
• Step 6: Extensibility and Trade-offs 11 min read Pro lesson

Module 21: Case Study: Career Trajectory Framework

Case Study: Career Trajectory Framework — 7 topics.

• The Problem 5 min read Pro lesson
• Step 1: Requirements 6 min read Pro lesson
• Step 2: Core Entities 10 min read Pro lesson
• Step 3: Classes and Relationships 8 min read Pro lesson
• Step 4: Code Implementation 9 min read Pro lesson
• Step 5: Edge Cases and Tests 12 min read Pro lesson
• Step 6: Extensibility and Trade-offs 13 min read Pro lesson

Module 22: Case Study: Tic-Tac-Toe

Case Study: Tic-Tac-Toe — 7 topics.

• The Problem 11 min read Pro lesson
• Step 1: Requirements 16 min read Pro lesson
• Step 2: Core Entities 17 min read Pro lesson
• Step 3: Classes and Relationships 17 min read Pro lesson
• Step 4: Code Implementation 17 min read Pro lesson
• Step 5: Edge Cases and Tests 13 min read Pro lesson
• Step 6: Extensibility and Trade-offs 15 min read Pro lesson

Module 23: Case Study: Snake and Ladder

Case Study: Snake and Ladder — 7 topics.

• The Problem 8 min read Pro lesson
• Step 1: Requirements 8 min read Pro lesson
• Step 2: Core Entities 11 min read Pro lesson
• Step 3: Classes and Relationships 12 min read Pro lesson
• Step 4: Code Implementation 12 min read Pro lesson
• Step 5: Edge Cases and Tests 10 min read Pro lesson
• Step 6: Extensibility and Trade-offs 10 min read Pro lesson

Module 24: Case Study: Chess Game

Case Study: Chess Game — 7 topics.

• The Problem 10 min read Pro lesson
• Step 1: Requirements 10 min read Pro lesson
• Step 2: Core Entities 14 min read Pro lesson
• Step 3: Classes and Relationships 14 min read Pro lesson
• Step 4: Code Implementation 14 min read Pro lesson
• Step 5: Edge Cases and Tests 11 min read Pro lesson
• Step 6: Extensibility and Trade-offs 10 min read Pro lesson

Module 25: Case Study: Connect Four

Case Study: Connect Four — 7 topics.

• The Problem 8 min read Pro lesson
• Step 1: Requirements 8 min read Pro lesson
• Step 2: Core Entities 11 min read Pro lesson
• Step 3: Classes and Relationships 12 min read Pro lesson
• Step 4: Code Implementation 12 min read Pro lesson
• Step 5: Edge Cases and Tests 10 min read Pro lesson
• Step 6: Extensibility and Trade-offs 10 min read Pro lesson

Module 26: Case Study: Card Game Framework

Case Study: Card Game Framework — 7 topics.

• The Problem 8 min read Pro lesson
• Step 1: Requirements 8 min read Pro lesson
• Step 2: Core Entities 14 min read Pro lesson
• Step 3: Classes and Relationships 14 min read Pro lesson
• Step 4: Code Implementation 14 min read Pro lesson
• Step 5: Edge Cases and Tests 10 min read Pro lesson
• Step 6: Extensibility and Trade-offs 11 min read Pro lesson

Module 27: Case Study: Simple Calculator

Case Study: Simple Calculator — 7 topics.

• The Problem 5 min read Pro lesson
• Step 1: Requirements 3 min read Pro lesson
• Step 2: Core Entities 5 min read Pro lesson
• Step 3: Classes and Relationships 6 min read Pro lesson
• Step 4: Code Implementation 6 min read Pro lesson
• Step 5: Edge Cases and Tests 4 min read Pro lesson
• Step 6: Extensibility and Trade-offs 3 min read Pro lesson

Module 28: Case Study: File System Tree

Case Study: File System Tree — 7 topics.

• The Problem 4 min read Pro lesson
• Step 1: Requirements 3 min read Pro lesson
• Step 2: Core Entities 4 min read Pro lesson
• Step 3: Classes and Relationships 5 min read Pro lesson
• Step 4: Code Implementation 6 min read Pro lesson
• Step 5: Edge Cases and Tests 3 min read Pro lesson
• Step 6: Extensibility and Trade-offs 3 min read Pro lesson

Module 29: Case Study: URL Shortener

Case Study: URL Shortener — 7 topics.

• The Problem 4 min read Pro lesson
• Step 1: Requirements 3 min read Pro lesson
• Step 2: Core Entities 4 min read Pro lesson
• Step 3: Classes and Relationships 6 min read Pro lesson
• Step 4: Code Implementation 6 min read Pro lesson
• Step 5: Edge Cases and Tests 5 min read Pro lesson
• Step 6: Extensibility and Trade-offs 6 min read Pro lesson

Module 30: Case Study: Movie Ticket Booking

Case Study: Movie Ticket Booking — 7 topics.

• The Problem 15 min read Pro lesson
• Step 1: Requirements 15 min read Pro lesson
• Step 2: Core Entities 19 min read Pro lesson
• Step 3: Classes and Relationships 18 min read Pro lesson
• Step 4: Code Implementation 19 min read Pro lesson
• Step 5: Edge Cases and Tests 18 min read Pro lesson
• Step 6: Extensibility and Trade-offs 20 min read Pro lesson

Module 31: Case Study: Snake Game

Case Study: Snake Game — 7 topics.

• The Problem 13 min read Pro lesson
• Step 1: Requirements 14 min read Pro lesson
• Step 2: Core Entities 20 min read Pro lesson
• Step 3: Classes and Relationships 16 min read Pro lesson
• Step 4: Code Implementation 14 min read Pro lesson
• Step 5: Edge Cases and Tests 13 min read Pro lesson
• Step 6: Extensibility and Trade-offs 16 min read Pro lesson

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