Low-Level Design Round

Overview

The LLD round tests a candidate's ability to translate requirements into clean, extensible object-oriented designs. Focus is on class relationships, interfaces, and design patterns not system-level architecture.

Round Structure

Time	Phase	What to Observe
0-5 min	Problem Introduction	Does candidate ask clarifying questions?
5-15 min	Requirements Gathering	Identifies actors, use cases, constraints
15-40 min	Design & Code	Class hierarchy, patterns, API contracts
40-50 min	Extensibility Discussion	How would design change with new requirements?
50-60 min	Trade-offs & Alternatives	Awareness of design trade-offs

Scoring Rubric

Dimension	1	2	3	4
Requirements	Jumps to code immediately	Asks few questions	Good requirement gathering	Exhaustive; identifies edge cases
Class Design	God class; no abstraction	Some separation but tightly coupled	Clean hierarchy; interfaces used	Elegant; easily extensible
Design Patterns	No awareness	Names patterns but misapplies	Applies 1-2 correctly	Multiple patterns; justifies choices
SOLID Principles	Violates most	Follows 1-2 accidentally	Consciously applies most	All principles; explains trade-offs
Extensibility	Design breaks with changes	Requires refactoring for changes	Handles most extensions cleanly	Open/Closed; plugin-ready

Popular LLD Problems

1. Parking Lot System
2. Elevator Design
3. Snake & Ladder Game
4. Online Bookstore / Library
5. Rate Limiter
6. Pub/Sub Messaging System
7. Splitwise / Expense Tracker
8. Vending Machine
9. Chess Game
10. File System (Linux-style)