The Parking Lot Problem

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Low-Level Design (LLD) is a cornerstone of software engineering interviews, especially for roles that demand strong system design skills. It involves breaking down high-level designs into detailed, implementable blueprints, ensuring that every component of a system is well-defined and functional. One of the most popular LLD interview questions is the Parking Lot Problem, which tests a candidate's ability to apply object-oriented design principles to a real-world scenario. In this newsletter, we’ll explore the importance of LLD, provide tips for mastering it, and walk you through solving the Parking Lot Problem step-by-step.

Why Low-Level Design (LLD) is Critical for Interviews

LLD is essential because it bridges the gap between high-level architecture and actual implementation. It ensures that the system is not only functional but also scalable, maintainable, and efficient. Interviewers focus on LLD to assess your ability to:

• Translate abstract ideas into concrete, actionable designs.

• Apply object-oriented principles like encapsulation, inheritance, and polymorphism.

• Handle real-world constraints such as scalability, performance, and edge cases.

Mastering LLD demonstrates your ability to think critically and design systems that work seamlessly in practice.

How to Prepare for Low-Level Design (LLD)

1. Understand Object-Oriented Design (OOD) Principles: Familiarize yourself with concepts like abstraction, inheritance, and design patterns (e.g., Factory, Singleton).

2. Practice Common Problems: Solve popular LLD problems like the Parking Lot, Library Management System, and Elevator System.

3. Focus on Scalability and Modularity: Design systems that can handle future growth and are easy to maintain.

4. Use UML Diagrams: Visualize your designs with class and sequence diagrams to communicate your ideas effectively.

5. Review Real-World Examples: Study how companies like Instagram or e-commerce platforms implement LLD in their systems.

The Parking Lot Problem: A Step-by-Step Solution

Problem Definition:

Design a parking lot system that:

• Supports multiple types of vehicles (e.g., motorcycles, cars, buses).

• Has different types of parking spots (e.g., compact, large, handicapped).

• Tracks vehicle entry and exit, and calculates parking fees based on the duration of stay.

• Handles edge cases like full capacity or invalid tickets.

Solution Approach:

1. Understand the Requirements:

   • Types of vehicles: Motorcycle, Car, Bus.

   • Types of spots: Compact, Large, Handicapped.

   • Payment system: Charges based on parking duration.

2. Identify Key Objects:

   • ParkingLot: Manages the entire parking facility.

   • ParkingSpot: Represents individual parking spots.

   • Vehicle: Base class for all vehicle types (e.g., Car, Motorcycle, Bus).

   • Ticket: Tracks entry time, exit time, and payment details.

   • PaymentSystem: Handles fee calculation and payment processing.

3. Class Design:

   class ParkingLot {
       List<ParkingSpot> parkingSpots;
       Map<Vehicle, Ticket> activeTickets;
   
       boolean parkVehicle(Vehicle vehicle);
       boolean unparkVehicle(Vehicle vehicle);
   }
   
   abstract class ParkingSpot {
       int spotId;
       boolean isAvailable;
       Vehicle currentVehicle;
   }
   
   class CompactSpot extends ParkingSpot {}
   class LargeSpot extends ParkingSpot {}
   class HandicappedSpot extends ParkingSpot {}
   
   abstract class Vehicle {
       String licensePlate;
   }
   
   class Car extends Vehicle {}
   class Motorcycle extends Vehicle {}
   class Bus extends Vehicle {}
   
   class Ticket {
       Vehicle vehicle;
       LocalDateTime entryTime;
       LocalDateTime exitTime;
       double fee;
   }
   
   class PaymentSystem {
       double calculateFee(Ticket ticket);
   }
   

4. Design Principles Applied:

   • Encapsulation: Each class handles its own data and behavior.

   • Inheritance: Vehicle and ParkingSpot are base classes with specific subclasses.

   • Modularity: Payment logic is separated into the PaymentSystem class for flexibility.

5. Algorithm for Parking a Vehicle:

   • Check for available spots of the appropriate type.

   • If a spot is available, assign the vehicle to the spot and generate a ticket.

   • If no spot is available, return an error indicating the lot is full.

6. Algorithm for Unparking a Vehicle:

   • Retrieve the ticket using the vehicle's details.

   • Calculate the parking fee using the PaymentSystem.

   • Mark the parking spot as available.

Frequently Asked Questions (FAQs) on LLD and the Parking Lot Problem

1. What is the difference between High-Level Design (HLD) and Low-Level Design (LLD)?
   Answer: HLD focuses on the overall architecture of the system, while LLD dives into the detailed design of individual components, including class structures and algorithms.

2. How do you handle scalability in the Parking Lot Problem?
   Answer: Use modular design and efficient data structures like hash maps to track available spots. Implement load balancing if the parking lot spans multiple locations.

3. How do you calculate parking fees?
   Answer: Use the Ticket class to track entry and exit times. The PaymentSystem calculates the fee based on the duration and vehicle type.

4. What design patterns are useful for the Parking Lot Problem?
   Answer: The Factory pattern can be used to create parking spots dynamically, and the Strategy pattern can handle different fee calculation methods.

Conclusion:

Low-Level Design is a vital skill for software engineers, and mastering it can significantly boost your interview performance. The Parking Lot Problem is an excellent example of how LLD principles can be applied to solve real-world challenges. By practicing problems like this and focusing on clear, modular designs, you’ll be well-prepared to tackle any LLD question in your next interview.

Remember, the key to success is practice and a solid understanding of object-oriented principles.

— Null Pointer Club Team