# Object-Oriented Design Interview: Parking Lot

"Design a parking lot" is the most common object-oriented design (OOD) question. The interviewer is checking whether you can turn vague requirements into clean classes and responsibilities, not whether you know a single right answer.

## Clarify first

- Multiple floors? Spot sizes (motorcycle, compact, large, EV charging)?
- Entry and exit gates, tickets, payment (hourly rate, flat fee)?
- Should it show free spots per floor on a display board?

## Core classes

```python
from enum import Enum
from datetime import datetime

class Size(Enum):
    SMALL = 1; MEDIUM = 2; LARGE = 3

class Vehicle:
    def __init__(self, plate, size): self.plate, self.size = plate, size

class Spot:
    def __init__(self, spot_id, size):
        self.id, self.size, self.vehicle = spot_id, size, None
    def fits(self, v): return self.vehicle is None and v.size.value <= self.size.value

class Ticket:
    def __init__(self, vehicle, spot):
        self.vehicle, self.spot, self.entered = vehicle, spot, datetime.now()

class ParkingLot:
    def __init__(self, floors): self.floors, self.active = floors, {}
    def park(self, v):
        for floor in self.floors:
            for spot in floor:
                if spot.fits(v):
                    spot.vehicle = v
                    t = Ticket(v, spot); self.active[v.plate] = t
                    return t
        return None                        # lot full
    def leave(self, plate, pricing):
        t = self.active.pop(plate); t.spot.vehicle = None
        return pricing.fee(t.entered, datetime.now())
```

## Design choices to talk about

- **Strategy pattern for pricing:** `HourlyPricing`, `FlatPricing`, weekend rates, all behind one `fee()` method.
- **Spot assignment strategy:** nearest to the entrance, or smallest spot that fits, behind an interface.
- **Faster lookup:** keep a free-spot set (or min-heap by distance) per size instead of scanning every spot.
- **Concurrency:** two gates can grab the same spot; lock the spot or use an atomic compare-and-set.

## What interviewers reward

Clear responsibilities (a ticket doesn't calculate fees), extensibility (adding EV spots touches one class), and asking about edge cases: lost tickets, full lot, a vehicle bigger than any spot.

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*Disclosure: I'm part of the [Linkjob AI](https://www.linkjob.ai/) team; we build an AI interview assistant.*