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Knowing which problems exist is one thing; knowing how to practice them systematically is another. Random practice is inefficient—you revisit problems too soon, skip foundational ones, and develop blind spots. This page provides a structured practice roadmap that maximizes your preparation efficiency.
The practice list is organized not by arbitrary order but by skill progression, dependency chains, and pattern coverage. Earlier problems build skills needed for later ones. Each problem is annotated with its category, primary patterns, estimated time, and what mastery looks like.
Treat this as your curriculum, not just a checklist. Complete problems in order, revisit them periodically, and track your progress systematically.
By following this practice roadmap, you will systematically cover all major LLD problem types, build skills in a logical progression, and develop the pattern recognition that enables you to tackle novel problems confidently. This is the path from beginner to interview-ready.
Before diving into the problems, understand the methodology behind this list and how to extract maximum value from your practice sessions.
The three phases of problem mastery:
Phase 1: Initial Attempt (45-60 min)
Phase 2: Analysis (20-30 min)
Phase 3: Revision (after 2-3 weeks)
You haven't mastered a problem until you can design it cleanly 3 weeks after your last attempt, without any reference. If you can't, you've memorized, not understood. Keep practicing until revision succeeds.
Start here regardless of experience level. These problems establish fundamental patterns and design thinking. Rushing past them creates shaky foundations that undermine later learning.
Completion target: Complete all Foundation problems within 2-3 weeks of starting LLD preparation.
| Problem | Category | Core Patterns | Time | Mastery Indicator | |
|---|---|---|---|---|---|
| F1 | Tic-Tac-Toe | Game | Strategy (win check), State (game flow) | 30-40 min | Clean separation of Board, Player, Game. Extensible to NxN. |
| F2 | Parking Lot (Basic) | Resource Management | Factory, Strategy (pricing) | 40-50 min | Vehicle hierarchy, Spot types, Ticket lifecycle. Simple pricing. |
| F3 | Vending Machine | Transaction | State (machine states), Strategy (payment) | 35-45 min | Complete state diagram with all transitions. Handle edge cases. |
| F4 | Stack Overflow (Basic) | Social/Content | Observer (notifications), Composite (threads) | 40-50 min | Question/Answer/Comment hierarchy. Basic voting. |
| F5 | Library Management (Basic) | Resource/Content | State (book status), Observer (notifications) | 40-50 min | Book, Member, Loan. Due date tracking. Simple fine calculation. |
Detailed problem guidance:
F1. Tic-Tac-Toe Why it's foundational: Introduces game modeling with minimal complexity. Focuses on clean separation between game state, players, and rules.
Key design decisions:
Extension to practice: Generalize to NxN board with K-in-a-row to win.
F2. Parking Lot (Basic) Why it's foundational: The canonical LLD introduction problem. Establishes Resource Management patterns that recur throughout the curriculum.
Key design decisions:
Extension to practice: Add multiple floors, entry/exit points.
F3. Vending Machine Why it's foundational: Pure state machine practice. Every state, every transition must be explicit.
Key design decisions:
Extension to practice: Multiple payment methods (cash, card, mobile).
F4. Stack Overflow (Basic) Why it's foundational: Introduces social/content patterns with manageable complexity. Combines content hierarchy with user interactions.
Key design decisions:
Extension to practice: Add tags, search, accepted answer highlighting.
F5. Library Management (Basic) Why it's foundational: Combines Resource Management (books are resources) with temporal logic (due dates, fines).
Key design decisions:
Extension to practice: Reservations, waiting lists, librarian roles.
Before moving to Intermediate, verify: Can you design each Foundation problem in under 35 minutes? Can you explain every design decision if challenged? Can you extend each problem with one new feature without restructuring? If yes, proceed. If no, repeat.
Intermediate problems introduce multi-component interactions, more complex state management, and algorithm integration. They build directly on Foundation patterns.
Completion target: Complete all Intermediate problems within 3-4 weeks after Foundation completion.
| Problem | Category | Core Patterns | Time | Mastery Indicator | |
|---|---|---|---|---|---|
| I1 | Snake and Ladder | Game | Strategy (dice), Observer (position changes) | 40-50 min | Board with snakes/ladders as modifiers. Multi-player support. |
| I2 | Movie Ticket Booking | Resource Management | State, Strategy, Factory | 45-55 min | Real-time seat locking. Show scheduling. Concurrency discussion. |
| I3 | Parking Lot (Full) | Resource Management | Factory, Strategy, Singleton, Observer | 50-60 min | Multi-floor, entry/exit tracking, dynamic pricing, notifications. |
| I4 | Hotel Booking System | Resource Management | Factory, Template Method, State | 50-60 min | Room types, date-range availability, reservation lifecycle. |
| I5 | ATM System | Transaction | State, Chain of Responsibility, Command | 50-60 min | Full state machine. Transaction types. Security considerations. |
| I6 | Shopping Cart | Transaction/Content | Observer, Strategy, Factory | 45-55 min | Cart operations, checkout flow, discount strategies. |
| I7 | Splitwise (Expense Sharing) | Social/Transaction | Strategy, Observer | 50-60 min | Expense tracking, debt simplification algorithm. |
| I8 | Car Rental System | Resource Management | Factory, State, Template Method | 50-60 min | Vehicle fleet, maintenance states, rental lifecycle. |
Detailed problem guidance:
I1. Snake and Ladder Skill progression from Foundation: Builds on Tic-Tac-Toe game patterns, adds board modifiers (snakes/ladders) and randomness (dice).
Key design decisions:
I2. Movie Ticket Booking Skill progression: Adds real-time concurrency concerns to Resource Management. Seats are resources with locking requirements.
Key design decisions:
I3. Parking Lot (Full) Skill progression: Extends Foundation's basic Parking Lot with enterprise-grade features.
Key design decisions:
I4. Hotel Booking System Skill progression: Date-range resource management. More complex availability queries than single-slot resources.
Key design decisions:
I5. ATM System Skill progression: Multi-step transactions with security and hardware abstraction.
Key design decisions:
I6. Shopping Cart Skill progression: E-commerce foundation. Complex pricing with discounts, taxes, promotions.
Key design decisions:
I7. Splitwise Skill progression: Graph-based debt simplification. Algorithm integration challenge.
Key design decisions:
I8. Car Rental System Skill progression: Fleet management with maintenance states. Multiple vehicle types and stations.
Key design decisions:
Intermediate problems should take 45-55 minutes each. If you're consistently exceeding 60 minutes, revisit Foundation problems. Speed comes from pattern recognition, and pattern recognition comes from Foundation mastery.
Advanced problems feature complex rule systems, sophisticated algorithms, or multi-subsystem coordination. They represent the upper end of typical LLD interview difficulty.
Completion target: Complete core Advanced problems (A1-A5) within 3-4 weeks after Intermediate. A6-A10 are stretch goals.
| Problem | Category | Core Patterns | Time | Mastery Indicator | |
|---|---|---|---|---|---|
| A1 | Chess | Game | Strategy, Command, Memento, State | 60-75 min | All piece movements. Check/checkmate detection. Special moves. |
| A2 | Elevator System | Infrastructure | State, Strategy, Observer | 55-65 min | Multi-elevator coordination. Scheduling algorithms. |
| A3 | Ride-Sharing (Uber) | Matching | Strategy, Observer, State, Factory | 55-65 min | Complete ride lifecycle. Matching algorithm. Pricing. |
| A4 | Food Delivery System | Matching | Strategy, Observer, State | 55-65 min | Restaurant-Order-Driver three-way. Order batching. |
| A5 | Airline Booking | Resource Management | Factory, Strategy, Composite | 55-65 min | Multi-leg flights. Seat classes. Ancillary services. |
| A6 | Social Media Feed (Twitter) | Social | Observer, Strategy, Iterator | 55-65 min | Follow graph. Feed generation. Trending. |
| A7 | Online Auction System | Transaction | State, Observer, Strategy | 50-60 min | Bid management. Auction states. Auto-bidding. |
| A8 | Music Streaming (Spotify) | Content | Iterator, Observer, Strategy | 50-60 min | Playlists. Recommendations. Playback state. |
| A9 | Task Scheduler | Infrastructure | Strategy, Observer, Command | 50-60 min | Job scheduling. Priority queues. Worker pools. |
| A10 | Rate Limiter | Infrastructure | Strategy, Singleton, Proxy | 45-55 min | Token bucket/sliding window algorithms. Per-client limits. |
Detailed problem guidance:
A1. Chess The complexity challenge: Chess has more rules than any other Game problem. Each piece type has unique movement. Check, checkmate, and stalemate detection require traversing possible moves. Special moves (castling, en passant, pawn promotion) add edge cases.
Key design decisions:
Mastery marker: Handling en passant and castling correctly shows deep understanding.
A2. Elevator System The algorithm challenge: Elevator scheduling is an optimization problem with multiple valid algorithms (FCFS, SCAN, LOOK). The design must accommodate different algorithms cleanly.
Key design decisions:
Mastery marker: Explaining trade-offs between scheduling algorithms (efficiency vs fairness).
A3. Ride-Sharing (Uber) The full-system challenge: Integrates real-time matching, geospatial concerns, complex state machine, and pricing strategies in one problem.
Key design decisions:
Mastery marker: Discussing how matching strategy changes under high demand.
A4. Food Delivery System The three-way matching challenge: Unlike Uber's two-party matching, food delivery matches restaurants, orders, and drivers.
Key design decisions:
Mastery marker: Explaining order batching trade-offs (efficiency vs freshness).
A5. Airline Booking The complex resource challenge: Multi-leg flights, connecting flights, seat classes, and ancillary services create a web of dependencies.
Key design decisions:
Mastery marker: Handling overbooking logic and upgrade waterfall.
In Advanced problems, you won't complete everything in 45 minutes. Prioritize: core entities and relationships first, key algorithms second, edge cases third. An interviewer would rather see a solid core design than a rushed attempt at completeness.
Expert problems go beyond typical LLD interviews into territory relevant for Staff, Principal, and Architect roles. They involve distributed system considerations, complex domain logic, or platform-level design. Complete these only after mastering Advanced tier.
Target audience: Candidates targeting Staff+ roles or those seeking comprehensive mastery.
| Problem | Category | Core Patterns | Time | Mastery Indicator | |
|---|---|---|---|---|---|
| E1 | Multi-Tenant SaaS Platform | Infrastructure | Factory, Strategy, Proxy, Decorator | 75-90 min | Tenant isolation. Resource quotas. Custom branding. |
| E2 | Trading Platform | Transaction | Command, Observer, Strategy, State | 75-90 min | Order matching. Position tracking. Risk limits. |
| E3 | Collaborative Editor (Google Docs) | Social/Infrastructure | Command, Observer, Memento | 75-90 min | Conflict resolution. Operational transform basics. |
| E4 | Distributed Cache System | Infrastructure | Strategy, Proxy, Observer | 60-75 min | Consistent hashing. Eviction policies. Replication. |
| E5 | Event Sourcing System | Infrastructure | Command, Observer, Memento | 60-75 min | Event store. Projections. Snapshots. |
| E6 | Feature Flag System | Infrastructure | Strategy, Factory, Proxy | 50-60 min | Flag targeting. Rollout strategies. A/B testing. |
| E7 | Notification System | Infrastructure | Observer, Strategy, Factory | 50-60 min | Multi-channel. Preferences. Rate limiting. |
| E8 | Search Engine (Basic) | Content/Infrastructure | Strategy, Iterator | 60-75 min | Indexing. Query parsing. Ranking. |
Why Expert problems matter:
Expert problems test whether you can design the foundational components that other systems depend on. They reveal understanding of:
When to attempt:
Only after:
Expert problems are not required for most interviews. They're for candidates who want complete mastery or are targeting roles where these exact problems (SaaS platforms, trading systems, collaboration tools) are relevant.
For Expert problems, focus on architectural decisions and trade-offs rather than exhaustive detail. In a 90-minute session, you might design core components, discuss key algorithms, and outline extension points—not implement everything. This mirrors how senior engineers actually work.
If you're targeting a specific company or domain, use these category-focused tracks instead of the general progression. Each track provides deep coverage of one problem category.
How to use category tracks:
Identify your target domain — What industry is the company in? What systems do they build?
Select the relevant track — Gaming company? Track A. Hotel/travel? Track B. Gig economy? Track C. Platform/infra? Track D.
Complete the track in order — Each track is a mini-curriculum with skill progression.
Supplement with general coverage — Don't skip other categories entirely; complete at least Foundation problems from all categories to maintain breadth.
For FAANG and large tech companies, general preparation (all categories) is better than deep track specialization. These companies ask varied problems. For domain-specific companies (Uber, Airbnb, Bloomberg), match your track to their domain.
Use this table to identify problems that target specific skills. If mock interviews or self-assessment reveal weaknesses, use targeted practice.
| Skill Gap | Target Problems | What to Focus On |
|---|---|---|
| State Machines | Vending Machine, ATM, Elevator, Ride-Sharing | Draw complete state diagrams before coding. Name every state, every transition. |
| Factory Pattern | Parking Lot (vehicle types), Shopping Cart (discounts), Notification System | Identify when runtime type selection is needed. Practice explaining when Factory beats constructors. |
| Strategy Pattern | Parking Lot (pricing), Elevator (scheduling), Ride-Sharing (matching) | Look for 'algorithm can vary' signals. Practice swapping strategies without changing clients. |
| Observer Pattern | Library (notifications), Social Feed, Real-time updates | Identify 'when X happens, notify Y' scenarios. Practice decoupling publishers from subscribers. |
| Relationship Modeling | Library, Chess, E-commerce | Diagram every relationship. State cardinality. Distinguish composition from aggregation. |
| Concurrency Awareness | Movie Tickets, Parking Lot, Hotel Booking | Identify race conditions. Discuss locking strategies. Know optimistic vs pessimistic. |
| Algorithm Integration | Elevator, Ride-Sharing, Splitwise | Practice encapsulating algorithms in Strategy classes. Discuss algorithm trade-offs. |
| Extension Design | All problems | After core design, add two features. If restructuring needed, design was insufficiently extensible. |
For each skill gap: 1) Select 2-3 target problems. 2) Before designing, explicitly state: 'I will focus on [skill].' 3) During design, pause whenever [skill] is relevant and verbalize your approach. 4) After design, evaluate: 'How well did I apply [skill]?' This deliberate focus accelerates improvement.
Time management is crucial. These schedules provide realistic timelines for different preparation scenarios.
These schedules assume quality completion. If you're rushing through problems without deep understanding, slow down. Completing 10 problems deeply beats completing 20 superficially. Adjust the schedule to your actual learning pace.
Track your progress systematically to identify patterns, measure improvement, and ensure comprehensive coverage.
| Problem | Date | Time | Completeness (1-5) | Revision Needed | Patterns Applied | Notes |
|---|---|---|---|---|---|---|
| Tic-Tac-Toe | Jan 5 | 32 min | 5 | No | Strategy, State | Clean design. Extended to 4x4. |
| Parking Lot | Jan 7 | 48 min | 4 | Yes (2 weeks) | Factory, Strategy | Missed multi-floor initially. |
| Chess | Jan 20 | 65 min | 3 | Yes (1 week) | Strategy, Command | Struggled with checkmate detection. |
What to track:
Self-assessment rubrics:
Level 1 (Struggling):
Level 2 (Developing):
Level 3 (Competent):
Level 4 (Proficient):
Level 5 (Expert):
You're interview-ready when: (1) All Foundation problems at Level 4+, (2) All Intermediate at Level 3+, (3) Core Advanced (A1-A5) at Level 3+, (4) Consistent timing under 50 minutes, (5) Mock interview feedback is positive.
You now have a complete, structured practice blueprint. This isn't just a list of problems—it's a curriculum designed for efficient skill building.
What's next:
The final page of this module covers mock interview recommendations—how to simulate real interview conditions, find practice partners, and get feedback that accelerates improvement. Practice alone builds skills; mock interviews build interview performance.
You have a complete practice roadmap: 23+ problems across four difficulty tiers, category-based tracks, skill-targeting strategies, schedule templates, and progress-tracking methods. Execute this plan systematically, and you'll be thoroughly prepared for LLD interviews.