Debug: Parking Lot - Two Cars, One Spot

Your task

Operations filed a bug report - it's README.md in the project, with the gate logs: one evening the lot sent a car to a spot that already had a car in it. The code that shipped is the project's ParkingLot.java, and Main.java replays that evening (in Python: parking_lot.py and main.py).

Find the root cause, fix it, and explain the root cause in the Root cause box under the editor - Submit needs it. Only the hidden tests decide whether the fix is accepted; your explanation is saved with the submission.

How the lot is supposed to work

• parkVehicle(vehicle) puts the vehicle in a free spot of its size - MOTORCYCLE in SMALL, CAR in MEDIUM, BUS in LARGE - and returns a new ticket, or null (None) if no spot of that size is free. In Python the methods are park_vehicle, unpark_vehicle and available_spots.
• unparkVehicle(ticket) frees the spot that ticket holds and returns true; a ticket it doesn't recognize returns false and changes nothing. A ticket is good for one exit.
• availableSpots(size) is the number of free spots of that size.

A good fix

• Is as small as the bug and keeps every constructor and public method signature.
• Fixes the cause, not just the evening in the report - the hidden suite checks the report's scenario, and everything the lot already did right, with other values.
• Comes with a root cause that says what in the code goes wrong and why, not just what you changed.

• Do not change the signatures of ParkingLot's constructor and public methods, or the classes in parkinglot/model/ - the hidden tests call them exactly as given.
• Assume single-threaded access.