Design a Parking Lot

In an AI-assisted coding round, design the core object model and APIs for a parking lot system. The system should support: - multiple floors - multiple spot types - vehicle types such as motorcycle, car, and truck - assigning a valid spot when a vehicle enters - issuing a parking ticket on entry - releasing the spot when the vehicle exits - calculating parking fees based on duration - querying current availability Explain the main classes, relationships, key methods, and how you would handle concurrent vehicle entry so that two vehicles cannot claim the same spot.