Why parking lot is a classic LLD problem - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the basic entity representing a vehicle in the parking lot system.

LLD

class Vehicle:
    def __init__(self, [1]):
        self.license_plate = license_plate

Drag options to blanks, or click blank then click option'

Alicense_plate

Bparking_fee

Centry_time

Dslot_number

Attempts:

3 left

💡 Hint

Common Mistakes

Using slot number as vehicle identifier

Using parking fee as vehicle attribute

The vehicle is identified by its license plate number, which is essential for tracking.

2fill in blank

medium

Complete the code to check if a parking slot is available.

LLD

def is_slot_available(self, [1]):
    return self.slots[slot_id] is None

Drag options to blanks, or click blank then click option'

Alicense_plate

Bentry_time

Cvehicle

Dslot_id

Attempts:

3 left

💡 Hint

Common Mistakes

Passing vehicle instead of slot ID

Using license plate to check slot availability

We check availability by slot ID, not by vehicle or other attributes.

3fill in blank

hard

Fix the error in the method that assigns a vehicle to a parking slot.

LLD

def park_vehicle(self, vehicle, [1]):
    if self.slots[slot_id] is None:
        self.slots[slot_id] = vehicle
        return True
    return False

Drag options to blanks, or click blank then click option'

Aentry_time

Bslot_number

Cslot_id

Dlicense_plate

Attempts:

3 left

💡 Hint

Common Mistakes

Using slot_number instead of slot_id

Using license_plate as slot key

The parameter name must match the key used to access the slot dictionary, which is slot_id.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps slot IDs to vehicles only if the slot is occupied.

LLD

occupied_slots = { [1]: [2] for [1] in self.slots if self.slots[[1]] is not None }

Drag options to blanks, or click blank then click option'

Aslot_id

Bvehicle

Cslot_number

Dcar

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for key and loop variable

Using slot_number instead of slot_id

We use slot_id as key and vehicle as value for occupied slots.

5fill in blank

hard

Fill all three blanks to implement a method that calculates total available slots by subtracting occupied slots from total slots.

LLD

def available_slots(self):
    total = len(self.slots)
    occupied = sum(1 for [1] in self.slots if self.slots[[2]] is not None)
    return total - [3]

Drag options to blanks, or click blank then click option'

Aslot_id

Bslot

Coccupied

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for loop and index

Subtracting wrong variable

We iterate over slot_id, check occupancy, and subtract occupied count from total.

Practice

1. Why is the parking lot problem considered a classic example in low-level design (LLD)?

easy

A. Because it requires complex database queries for vehicle tracking

B. Because it is only about calculating parking fees

C. Because it focuses mainly on front-end user interface design

D. Because it involves managing different types of vehicles and parking spots with clear rules

Solution

1. Step 1: Understand the core challenge of parking lot design

   The problem requires managing different vehicle types (cars, bikes, trucks) and matching them to appropriate parking spots with specific rules.

2. Step 2: Identify why this fits LLD learning

   This involves object modeling, resource allocation, and rule enforcement, which are key LLD concepts.

3. Final Answer:

   Because it involves managing different types of vehicles and parking spots with clear rules -> Option D

4. Quick Check:

   Parking lot = resource allocation + object modeling [OK]

Hint: Focus on resource management and object rules in parking lot [OK]

Common Mistakes:

• Thinking it's mainly about UI or fees
• Confusing with database or front-end problems
• Ignoring the variety of vehicle and spot types

2. Which of the following is the correct way to represent a parking spot in a low-level design for a parking lot?

easy

A. function ParkingSpot() { return spotNumber + spotType; }

B. class ParkingSpot { int spotNumber; String spotType; boolean isOccupied; }

C. var ParkingSpot = [spotNumber, spotType, isOccupied];

D. ParkingSpot = spotNumber * spotType * isOccupied;

Solution

1. Step 1: Identify proper class structure for parking spot

   A parking spot should be modeled as a class with attributes like spot number, type, and occupancy status.

2. Step 2: Evaluate options for correctness

   class ParkingSpot { int spotNumber; String spotType; boolean isOccupied; } defines a class with clear attributes, suitable for LLD. Others are either functions, arrays, or invalid expressions.

3. Final Answer:

   class ParkingSpot { int spotNumber; String spotType; boolean isOccupied; } -> Option B

4. Quick Check:

   Class with attributes = correct parking spot model [OK]

Hint: Use classes with clear attributes for entities [OK]

Common Mistakes:

• Using arrays or functions instead of classes
• Mixing data types incorrectly
• Not including occupancy status

3. Given this simplified code snippet for a parking lot system, what will be the output?

class ParkingLot:
    def __init__(self):
        self.spots = {"car": 2, "bike": 1}
    def park_vehicle(self, vehicle_type):
        if self.spots.get(vehicle_type, 0) > 0:
            self.spots[vehicle_type] -= 1
            return "Parked"
        else:
            return "Full"

lot = ParkingLot()
print(lot.park_vehicle("car"))
print(lot.park_vehicle("car"))
print(lot.park_vehicle("car"))

medium

A. Parked
Parked
Full

B. Full
Full
Full

C. Parked
Full
Parked

D. Error due to missing bike spots

Solution

1. Step 1: Analyze initial spot counts and park_vehicle calls

   Initially, car spots = 2. First call parks a car, spots become 1. Second call parks another car, spots become 0. Third call finds no spots left.

2. Step 2: Determine output for each print statement

   First two prints output "Parked", third outputs "Full" because no spots remain.

3. Final Answer:

   Parked Parked Full -> Option A

4. Quick Check:

   Spot count decreases, last attempt fails [OK]

Hint: Track spot count decrement per park call [OK]

Common Mistakes:

• Assuming infinite spots
• Ignoring spot decrement
• Confusing vehicle types

4. In this parking lot design code, what is the bug causing incorrect spot allocation?

class ParkingLot:
    def __init__(self):
        self.spots = {"car": 2}
    def park_vehicle(self, vehicle_type):
        if self.spots[vehicle_type] > 0:
            self.spots[vehicle_type] = 0
            return "Parked"
        else:
            return "Full"

lot = ParkingLot()
print(lot.park_vehicle("car"))
print(lot.park_vehicle("car"))

medium

A. The park_vehicle method does not return any value

B. The spots dictionary is missing bike spots

C. The spot count is set to 0 instead of decrementing by 1

D. The vehicle_type key is misspelled

Solution

1. Step 1: Review spot count update logic

   The code sets spots[vehicle_type] = 0 directly instead of subtracting 1. With 2 initial car spots, first park sets it to 0 (should be 1).

2. Step 2: Understand impact on multiple park calls

   First park: "Parked", spots=0. Second park: "Full" but should succeed if decremented properly.

3. Final Answer:

   The spot count is set to 0 instead of decrementing by 1 -> Option C

4. Quick Check:

   Spot count update should decrement, not assign zero [OK]

Hint: Always decrement spot count, don't assign zero directly [OK]

Common Mistakes:

• Ignoring decrement logic
• Assuming spots dictionary must have all vehicle types
• Overlooking return statements

5. You are designing a parking lot system that must handle cars, bikes, and trucks with different spot sizes. Which design approach best supports scalability and maintainability?

hard

A. Create separate classes for each vehicle and spot type with a common interface for parking logic

B. Use a single class for all vehicles and spots with many if-else checks for types

C. Store all vehicles in a single list and assign spots randomly

D. Design only for cars first, then add bikes and trucks later without changing code

Solution

1. Step 1: Consider object-oriented design principles

   Using separate classes with a common interface allows clear modeling of different vehicle and spot types and their behaviors.

2. Step 2: Evaluate scalability and maintainability

   This approach supports adding new vehicle types easily and keeps code clean, unlike monolithic classes or random assignments.

3. Final Answer:

   Create separate classes for each vehicle and spot type with a common interface for parking logic -> Option A

4. Quick Check:

   Use OOP with interfaces for scalable parking lot design [OK]

Hint: Use separate classes and interfaces for each type [OK]

Common Mistakes:

• Using one class with many conditions
• Ignoring scalability needs
• Delaying design for other vehicle types