4. LLD Interview Problems

A curated list of 20 common LLD interview problems, ranked by Probability of being asked in SDE 2 interviews.

Legend:

• Very High (80-100%): Must prepare. Asked in almost every LLD round.

• High (60-80%): Frequently asked.

• Medium (20-60%): Domain-specific or less common.

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The Master List (Ranked)

Rank	Problem	Probability	Difficulty	Key Concepts
1	Design Parking Lot	95%	Medium	Singleton, Factory, Strategy, Concurrency
2	Design Rate Limiter	90%	Medium	Token Bucket, Thread-Safety, Decorator
3	Design Tic-Tac-Toe	85%	Easy	Game Loop, 2D Arrays, Validation Logic
4	Design Vending Machine	85%	Medium	State Pattern, State Machine, Money Handling
5	Design Splitwise	80%	Medium	Graph Simplification, Strategy (Splits), User/Group
6	Design Snake & Ladder	75%	Medium	Game Entity, Observer, Strategy (Dice)
7	Design Elevator System	75%	Medium	State Pattern, Scheduling Algorithms (SCAN/LOOK)
8	Design Comment System	70%	Medium	Materialized Path (Trees), Recursion, Database Design
9	Design Hotel Management	70%	Medium	Booking Factory, Date Concurrency, Singleton
10	Design Cache (LRU/LFU)	65%	Hard	Doubly Linked List + HashMap, Generics
11	Design Locker Service (Amazon)	65%	Medium	Geo-hashing, Locker Size Matching, Inheritance
12	Design Coupon System	60%	Medium	Composite Pattern, Chain of Responsibility
13	Design Mentorship Platform ‍	55%	Medium	Booking availability, Conflict resolution
14	Design Logger Library	50%	Easy	Chain of Responsibility, Singleton, Sink Strategy
15	Design Minesweeper	45%	Medium	Flood Fill (DFS/BFS), Recursion
16	Design File System / S3	40%	Hard	Composite Pattern, Metadata vs Data, Permissions
17	Design Search Engine	35%	Hard	Inverted Index, Tries, Tokenization
18	Design Tetris	30%	Hard	Matrix Rotation, Factory Pattern, Game Loop
19	Design Version Control (Git)	25%	Hard	Graph (DAG), Hashing (SHA-1), Merkle Tree
20	Design Tunneling Service	20%	Hard	Reverse Proxy, Socket Programming
21	Design Text Editor	20%	Hard	Gap Buffer/Rope, Command Pattern, Undo/Redo
22	Design Download Manager	15%	Hard	Multi-threading, HTTP Ranges, File Merging
23	Design Unlock Pattern	10%	Medium	DFS/Backtracking, Validation Logic

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Preparation Strategy

1. Tier 1 (Fire ): Implement these from scratch 3 times. These are your bread and butter.

• Parking Lot, Rate Limiter, Vending Machine, Tic-Tac-Toe, Splitwise.

1. Tier 2 (Green ): Understand the Class Diagram and Key Design Pattern.

• Elevator, Snake & Ladder, Comments, Hotel, LRU Cache.

1. Tier 3 (Yellow ): Read the code to understand specific algorithms (e.g. QuadTree for Locker, Tries for Search).

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Note: Problems marked with links are completed.