Object Design · LLD Interview Questions — Hard

Answer: Design an in-memory, thread-safe rate limiter supporting multiple rate limiting algorithms per user or API client.

• Solution Link: https://blog.algomaster.io/p/rate-limiting-algorithms-explained-with-code
• Key Design Patterns: Strategy Pattern (TokenBucketStrategy, LeakyBucketStrategy, SlidingWindowCounterStrategy), Factory Pattern (RateLimiterFactory).
• Core Classes: RateLimiter, RateLimitConfig (capacity, refillRate, windowSeconds), TokenBucket, ClientRequest.

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Answer: Design an in-memory key-value cache supporting $O(1)$ time complexity for both get(key) and put(key, value) with automatic eviction of the least recently used element upon reaching capacity.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/lru-cache.md
• Data Structures Used: Hash Map (for $O(1)$ key lookup) + Custom Doubly Linked List (for $O(1)$ node removal and head insertion).
• Core Classes: LRUCache, Node (key, val, prev, next), DoublyLinkedList (addToHead(), removeNode(), moveToHead(), removeTail()).

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Answer: Design a project collaboration board supporting workspaces, boards, customizable columns/lists, task cards with assignees, labels, and activity history.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/task-management-system.md
• Key Design Patterns: Observer Pattern (Notify assignees on task changes), State Pattern (TaskStatus: TODO, IN_PROGRESS, IN_REVIEW, DONE), Composite Pattern.
• Core Classes: User, Workspace, Board, TaskList / Column, TaskCard, Comment, ActivityLog.

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Answer: Design a university portal managing courses, professors, prerequisites validation, enrollment caps, student waitlists, and GPA grade calculation.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/course-registration-system.md
• Key Design Patterns: Observer Pattern (Waitlist notifications), Strategy Pattern (PrerequisiteVerificationStrategy), Singleton Pattern.
• Core Classes: Course, CourseSection, Student, Professor, Registration, WaitlistQueue, GradeRecord.

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Answer: Design an automated four-way intersection controller supporting emergency vehicle preemption, dynamic sensor-based green duration, and pedestrian crossing requests.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/traffic-signal.md
• Key Design Patterns: State Pattern (SignalState: RED, YELLOW, GREEN), Observer Pattern (Emergency vehicle sensors), Strategy Pattern (CycleTimingStrategy).
• Core Classes: TrafficController, Intersection, TrafficLight, Road, VehicleSensor, PedestrianButton.

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