Parking Lot

PayPal's "Parking Lot" interview question is a classic low-level system design (LLD) problem often tagged for backend/web roles, focusing on object-oriented design rather than high-level scalability. It requires modeling a multi-level parking structure with various vehicle types and spots.

Problem Statement

Design a parking lot system that manages vehicle entry, parking assignment, and exit across multiple levels. The system must support different vehicle types (e.g., motorcycle, car, truck), spot sizes (small, compact, large), and efficient spot allocation based on availability. Core operations include parking a vehicle (assigning the nearest available matching spot), unparking (releasing a spot and issuing a ticket/fee), and querying availability. Assume no payments, gates, or UI—focus on classes, relationships, and logic.[2][5]

Key Requirements

• Functional: Park/unpark vehicles; find spots by type; track occupancy per level/spot type; generate unique spot IDs and tickets.
• Non-functional: O(1) or near-constant time for park/unpark; handle multiple levels (e.g., 5 levels) and spots per type (e.g., 5 small, 5 large per level).
• Entities: ParkingLot, ParkingLevel, ParkingSpot, Vehicle (with types like Small, Large), Ticket.
• Out of scope: Payments, hardware (cameras/gates), high-level scaling (use simple lists/maps).[3][5][2]

Input/Output Examples

From common implementations (e.g., Python class designs in interviews):

Park a Vehicle: Input: park_vehicle("small") # or vehicle type/size Output: (spot_id=1, level=0) # Nearest available small spot on level 0 After multiple calls: park_vehicle("small") → (1, 0) park_vehicle("small") → (2, 0) park_vehicle("large") → No available spots (if full)

Unpark a Vehicle: Input: release_parking(spot_id=1, level_id=0) Output: "Released parking at spot 1, level 0" # Spot freed

Full Example Usage: lot = ParkingLot(levels=5, spots_per_type=5, types=["small", "large"]) lot.park_vehicle("small") # Returns (1, 0) lot.release_parking(1, 0) # Frees spot lot.get_free_count("small") # Returns updated count, e.g., 4 initially per level Simulates real flow: allocate → occupy → release → reallocate.[2]

Constraints

• Up to 10 levels, 100 spots per type per level.
• Vehicle types: 2-4 (e.g., small for motorcycles/cars, large for trucks).
• Spot types match vehicle sizes (small fits small/medium; large fits all).
• No concurrent access specified (single-threaded logic).
• Edge cases: Full lot, invalid spot ID, nearest-level allocation (scan levels sequentially).[5][2]