Practice LLD — with a verdict
Each problem gives you the requirements and a class skeleton, then grades your design against hidden behavioral tests on the same judge as the DSA track.
Module 0 — Foundations
- 01The Four Pillars, As Interviewers Test Them
12 min + quiz
- 02SOLID I: SRP & OCP with Refactors
13 min + quiz
- 03SOLID II: LSP, ISP, DIP
13 min + quiz
- 04Patterns I: Factory, Builder, Singleton
12 min + quiz
- 05Patterns II: Strategy, Observer, Command, State
13 min + quiz
- 06Patterns III: Decorator, Adapter, Facade + Choosing Under Pressure
13 min + quiz
- 07Walkthrough: Design a Parking Lot
10 min + quiz
- 08Walkthrough: Design an LRU Cache
10 min + quiz
- 09Walkthrough: Design a Rate Limiter
10 min + quiz
- 10Walkthrough: Design an Elevator System
10 min + quiz
- 11Walkthrough: Design a Notification Service
10 min + quiz
- 12Walkthrough: Design Splitwise
10 min + quiz
Learn LLD, step by step
0 / 11 modules doneShort, interactive modules that take you from object basics to worked system designs. Each tab is a few minutes of teaching with a code artifact and a quick check, so you build the pattern-recognition an interviewer looks for.
Foundations
Object modeling and the principles interviewers grade on.
Composition Over Inheritance
Why deep class hierarchies collapse under changing requirements, and how to model behavior with composition instead.
10 min4 checksSOLID, The Way Interviewers Grade It
The five design principles that separate a passing machine-coding solution from a mess — each with the refactor it forces.
12 min5 checksClass Relationships & UML for Machine Coding
Turn requirements into a class diagram fast — read and draw the four relationships before you write a line of code.
11 min7 checks
Design Patterns
The reusable solutions that show up in machine-coding rounds.
Creational Patterns That Earn Their Keep
Factory, Builder, Singleton, Prototype — when object creation deserves its own abstraction, and when it doesn't.
12 min7 checksStructural Patterns for Clean Object Graphs
Adapter, Decorator, Composite, Facade, Proxy — compose objects so structure stays flexible as requirements grow.
12 min7 checksBehavioral Patterns: Modeling How Objects Collaborate
Strategy, Observer, State, Command, Template Method — the collaboration patterns most machine-coding problems actually need.
12 min10 checks
Concurrency
Shared state, locks, and thread-safe designs.
Craft
How to actually run a timed machine-coding round.
Worked Systems
Build a full design end to end, one decision at a time.
Build It: Parking Lot
Design a multi-level parking lot end to end — entities, spot allocation, pricing, and thread safety — one decision at a time.
14 min7 checksBuild It: LRU Cache
Design an O(1) LRU cache from the API down to the HashMap + doubly linked list, then make it production-ready.
13 min9 checksBuild It: Rate Limiter
Design a rate limiter from requirements through fixed window, sliding window, and token bucket — and know which to pick.
13 min7 checks
LLD — code the design
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