CareerCup

class ParkingLot {
private ArrayList<Slot> slots;
int totalFreeSlots;
}
abstract class Slot {
int slotid;
int floorNo;
boolean occupied;
int getHeight();
int getWidth();
}

class CompactSlot extends Slot {
int width=xx;
int height=yy;
}

class LargeSlot extends Slot {
int width=xx;
int height=yy;
}

class DisabledSlot extends Slot {
int width=xx;
int height=yy;
}

Class Building{
List <Floor> floors;

//Member methods
}

class Floor{
List <ParkingLot> parkingLots;

//member Methods
}

class ParkingLot{
Boolean exists;

//member methods
}

Use of Singleton pattern makes sense here since a common parking lot will be shared by all the vehicles, so you might need to incorporate that in your code.
There's also a scope of using Factory method pattern for ParkingLot object depending on the size of the vehicle (although, it might not be necessary)
I can also think of time stamps?