Object Design · LLD Interview Questions — Medium

Answer: Design a multi-floor parking lot system supporting different vehicle types, automated ticket issuance, spot allocation strategies, and payment calculation.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/parking-lot.md
• Key Design Patterns: Singleton (ParkingLot instance), Strategy Pattern (ParkingSpotAssignmentStrategy, FeeCalculationStrategy), Factory Pattern (VehicleFactory).
• Core Classes: ParkingLot, ParkingFloor, ParkingSpot (subclasses: CompactSpot, LargeSpot, HandicappedSpot), Vehicle (subclasses: Car, Truck, Motorcycle), Ticket, Payment.
• Concurrency Handling: Thread-safe synchronized slot booking using atomic compare-and-swap or reentrant locks to prevent double-booking the same spot.

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Answer: Design an elevator controller managing multiple elevator cars across a multi-story building, processing internal destination presses and external floor calls with optimal scheduling.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/elevator-system.md
• Key Design Patterns: State Pattern (ElevatorState: MOVING_UP, MOVING_DOWN, IDLE), Strategy Pattern (LOOK/SCAN Dispatcher Algorithm), Observer Pattern (Floor buttons notify Dispatcher).
• Core Classes: ElevatorController, ElevatorCar, Door, InternalButtonPanel, ExternalCallButton, Request (sourceFloor, destinationFloor, direction).

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Answer: Design a state-driven vending machine supporting item selection, cash/coin insertion, change return, and out-of-stock management.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/vending-machine.md
• Key Design Patterns: State Pattern (IdleState, HasMoneyState, DispensingState, SoldOutState), Inventory Management Pattern.
• Core Classes: VendingMachine, State interface, Inventory, Item, Coin/Note, Dispenser.

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Answer: Design an automated coffee dispenser supporting customizable recipes, dynamic toppings/condiments, and ingredient inventory tracking.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/coffee-vending-machine.md
• Key Design Patterns: Decorator Pattern (Base Coffee decorated with MilkDecorator, SugarDecorator, CaramelDecorator), Factory Pattern (CoffeeFactory), State Pattern.
• Core Classes: Coffee interface, Espresso, Latte, Cappuccino, CondimentDecorator, IngredientInventory, PaymentProcessor.

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Answer: Design an Automated Teller Machine (ATM) supporting card validation, PIN verification, cash withdrawal with currency denomination dispensing, balance inquiries, and transaction rollback.

• Solution Link: https://github.com/ashishps1/awesome-low-level-design/blob/main/problems/atm.md
• Key Design Patterns: State Pattern (IdleState, CardInsertedState, PinEnteredState, CashDispensedState), Chain of Responsibility Pattern (Cash Dispenser: 2000 $\to$ 500 $\to$ 100 notes).
• Core Classes: ATM, CardReader, CashDispenser, BankService, Account, Transaction.

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