This is actually a very common LLD interview question.

The interviewer usually doesn't expect you to use every design pattern. They expect you to identify where each pattern fits naturally.

Car Rental System (Uber/Ola/Zoomcar style)

High Level Flow

                    +----------------+
                    |      User      |
                    +----------------+
                            |
                     Search Car
                            |
                            v
                 +------------------+
                 | CarRentalSystem   |
                 +------------------+
                  |        |       |
                  |        |       |
                  |        |       |
                  v        v       v

           +---------+ +---------+ +---------+
           |  Car    | | Booking | | Payment |
           +---------+ +---------+ +---------+
                  |          |           |
                  |          |           |
                  v          v           v

         +-----------+ +-----------+ +-----------+
         | Sedan     | | Active    | | CreditCard|
         | SUV       | | Completed | | UPI       |
         | Hatchback | | Cancelled | | Wallet    |
         +-----------+ +-----------+ +-----------+

---

Design Patterns We Can Use

1. Factory Pattern ⭐⭐⭐

Problem

Many car types exist.

Sedan
SUV
Hatchback
Luxury

Don't create objects everywhere.

Diagram

Admin/User
    |
    v
CarFactory
    |
    +--> Sedan
    |
    +--> SUV
    |
    +--> Hatchback

Rule

Factory = "Give me a car"

Example

car = CarFactory.create("SUV")

---

2. Singleton Pattern ⭐⭐⭐

Problem

Entire application should have only one rental system.

User A ----\
User B -----> CarRentalSystem
User C ----/

Only one system

Use

CarRentalSystem

stores:

• all users
• all bookings
• all cars

Rule

Singleton = One rental system for everyone

---

3. Builder Pattern ⭐⭐⭐

Problem

Booking has many optional fields.

Pickup location
Drop location
Insurance
GPS
Baby seat
Extra driver

Instead of:

Booking(a,b,c,d,e,f,g,h)

Build step by step.

Diagram

User

|

+--> Pickup

|

+--> Drop

|

+--> Insurance

|

+--> GPS

|

+--> Build

      |

      v

   Booking

Rule

Builder = Build booking step by step

---

4. Strategy Pattern ⭐⭐⭐

Problem

Different payment methods.

Credit Card
UPI
Wallet
Net Banking

Diagram

User
 |
 v
PaymentService
 |
 +--> UPI Strategy
 |
 +--> Card Strategy
 |
 +--> Wallet Strategy

Rule

Strategy = Change behavior at runtime

---

5. Observer Pattern ⭐⭐⭐

Problem

Notify users.

Booking confirmed

Car arriving

Trip started

Trip completed

Diagram

Booking System
       |
       +--> User
       |
       +--> Driver
       |
       +--> Admin

Rule

Observer = One changes, many get notified

---

6. Facade Pattern ⭐⭐

Hide complexity.

Instead of calling:

Search Service

Booking Service

Payment Service

Notification Service

Expose one interface.

User
 |
 v
RentalFacade
 |
 +--> Search
 |
 +--> Booking
 |
 +--> Payment
 |
 +--> Notification

Rule

Facade = One entry point

---

Final LLD Diagram

                           User
                             |
                             |
                             v
                  +-------------------+
                  |   RentalFacade    |
                  +-------------------+
                    |      |       |
                    |      |       |
                    v      v       v

             SearchCar  Booking  Payment

                    |       |       |
                    |       |       |

                    v       v       v

            +------------+  +----------------+
            | CarFactory |  | BookingBuilder |
            +------------+  +----------------+
               |                |
         +-----+-----+          |
         |     |     |          |
       Sedan  SUV Hatchback     |
                                |
                                v
                            Booking


Payment
   |
   +--> UPI Strategy
   +--> Card Strategy
   +--> Wallet Strategy


Booking Updates
       |
       +--> User
       +--> Driver
       +--> Admin
       (Observer)


Entire App
       |
       v
CarRentalSystem (Singleton)

Interview Cheat Sheet

Component	Pattern	Why
Car creation	Factory	Many car types
Main system	Singleton	Only one system
Booking creation	Builder	Many optional fields
Payment	Strategy	Multiple payment algorithms
Notifications	Observer	Notify many users
Main API	Facade	Hide complexity

For LLD interviews, whenever you hear Car Rental, Food Delivery, Hotel Booking, Library Management, Parking Lot, first think:

Objects -> Factory

One global system -> Singleton

Complex object -> Builder

Switch behavior -> Strategy

Notifications -> Observer

Hide complexity -> Facade

This shortcut solves 70-80% of interview questions.