LLD interview expectations - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Key Focus Areas in LLD Interviews

Which of the following is NOT typically a key focus area in a Low-Level Design (LLD) interview?

A Writing efficient SQL queries for data retrieval

B Ensuring modularity and reusability in code

C Designing classes and their relationships

D Applying design patterns appropriately

💡 Hint

Think about what LLD interviews mainly test: code structure and design, not database querying.

LLD interviews focus on designing classes, interfaces, and applying design patterns. Writing SQL queries is usually part of database or backend interviews, not LLD.

❓ Architecture

intermediate

Class Diagram for a Simple Library System

In a Low-Level Design interview, you are asked to design a simple library system. Which class relationship best represents the connection between 'Book' and 'Author'?

A Composition: Book is composed of Authors

B Inheritance: Author inherits from Book

C Aggregation: Author contains Books

D Association: Book has one or more Authors

💡 Hint

Consider if a Book can exist without an Author and vice versa.

Books and Authors have an association relationship because a book can have multiple authors, and authors can write multiple books. Neither inherits from the other, and composition or aggregation do not fit well here.

❓ scaling

advanced

Handling Increased Load in LLD Designs

You designed a notification system in your LLD interview. The interviewer asks how you would modify your design to handle a sudden increase in users sending notifications simultaneously. Which approach is the best to scale your design?

A Store all notifications in a single database table with no indexing

B Use a message queue to decouple notification sending from processing

C Add more methods to the Notification class to handle concurrency

D Make the Notification class static to share state across threads

💡 Hint

Think about how to handle many requests without blocking or losing data.

Using a message queue helps to buffer and process notifications asynchronously, improving scalability and reliability. Other options either do not address concurrency properly or can cause bottlenecks.

❓ tradeoff

advanced

Tradeoff Between Flexibility and Complexity

In an LLD interview, you propose a design with many interfaces and abstract classes to allow future flexibility. The interviewer asks about the tradeoff of this approach. What is the main downside?

A It makes the system run faster

B It eliminates the need for unit testing

C It reduces code readability and increases complexity

D It decreases modularity

💡 Hint

More abstraction can make code harder to understand and maintain.

While interfaces and abstractions improve flexibility, they can also make the codebase more complex and harder to read, which is a common tradeoff in design.

❓ estimation

expert

Estimating Capacity for a Chat Application

You designed a chat application in your LLD interview. The interviewer asks: If your system expects 1 million active users sending an average of 10 messages per minute, how many messages per second must your system handle?

A Approximately 166,667 messages per second

B Approximately 10,000 messages per second

C Approximately 166 messages per second

D Approximately 600,000 messages per second

💡 Hint

Calculate total messages per minute and convert to per second.

1 million users * 10 messages/minute = 10 million messages/minute. Dividing by 60 seconds gives about 166,667 messages per second.

Practice

1. What is the main focus of a Low-Level Design (LLD) interview?

easy

A. Choosing cloud providers and infrastructure

B. Estimating system capacity and scaling

C. Designing detailed components and their interactions

D. Writing complex SQL queries

Solution

1. Step 1: Understand LLD interview scope

   LLD interviews focus on designing small parts of a system clearly and in detail.

2. Step 2: Compare options with LLD focus

   Estimating capacity and scaling, choosing cloud providers and infrastructure, and writing database queries relate to other areas, not the main focus of LLD interviews.

3. Final Answer:

   Designing detailed components and their interactions -> Option C

4. Quick Check:

   LLD = detailed component design [OK]

Hint: LLD = detailed design of small system parts [OK]

Common Mistakes:

• Confusing LLD with high-level system design
• Thinking LLD focuses on infrastructure
• Mixing LLD with database query writing

2. Which of the following is the correct way to represent a relationship in a class diagram during an LLD interview?

easy

A. Using arrows to show inheritance and lines for associations

B. Using SQL queries to define relationships

C. Using cloud icons to represent services

D. Using flowcharts to show data flow

Solution

1. Step 1: Identify class diagram conventions

   Class diagrams use arrows for inheritance and lines for associations between classes.

2. Step 2: Eliminate unrelated options

   SQL queries, cloud icons, and flowcharts are not used to represent class relationships in LLD diagrams.

3. Final Answer:

   Using arrows to show inheritance and lines for associations -> Option A

4. Quick Check:

   Class diagram = arrows + lines [OK]

Hint: Class diagrams use arrows and lines for relationships [OK]

Common Mistakes:

• Confusing class diagrams with flowcharts
• Using database or cloud symbols incorrectly
• Ignoring inheritance arrows

3. Consider this simplified class design snippet in an LLD interview:

class User {
  int id;
  String name;
  List<Order> orders;
}

class Order {
  int orderId;
  double amount;
}

What is the relationship between User and Order?

medium

A. Inheritance

B. Aggregation

C. Composition

D. No relationship

Solution

1. Step 1: Analyze the class fields

   User has a list of Orders, meaning User contains Orders but Orders can exist independently.

2. Step 2: Determine relationship type

   Since Orders can exist without User, this is aggregation, not composition or inheritance.

3. Final Answer:

   Aggregation -> Option B

4. Quick Check:

   User "has" Orders = Aggregation [OK]

Hint: List field means aggregation, not inheritance [OK]

Common Mistakes:

• Confusing aggregation with composition
• Thinking it's inheritance due to class names
• Ignoring independence of Order objects

4. You designed a class diagram but forgot to show the multiplicity on associations. What issue might this cause in an LLD interview?

medium

A. Ambiguity about how many objects relate to each other

B. Syntax error in the diagram

C. The system will not compile

D. No impact, multiplicity is optional

Solution

1. Step 1: Understand multiplicity role

   Multiplicity shows how many instances of one class relate to another, clarifying design details.

2. Step 2: Consequence of missing multiplicity

   Without it, the design is unclear about object counts, causing ambiguity.

3. Final Answer:

   Ambiguity about how many objects relate to each other -> Option A

4. Quick Check:

   Missing multiplicity = unclear relationships [OK]

Hint: Multiplicity clarifies object counts in relationships [OK]

Common Mistakes:

• Thinking diagram syntax breaks without multiplicity
• Assuming multiplicity is always optional
• Confusing multiplicity with inheritance

5. During an LLD interview, you are asked to design a simple library system with Books and Members. Which design choice best follows LLD interview expectations?

hard

A. Design a single class Library with all methods and no separation

B. Use complex inheritance hierarchies for Book types without explaining choices

C. Focus only on database schema without class diagrams

D. Create classes Book and Member with clear attributes and a borrowing relationship using aggregation

Solution

1. Step 1: Identify good LLD design practices

   LLD expects clear classes with attributes and simple relationships like aggregation.

2. Step 2: Evaluate options against LLD expectations

   Creating classes Book and Member with clear attributes and a borrowing relationship using aggregation matches best by defining classes and relationships clearly. A single class lacks separation, database schema skips class diagrams, and unexplained complex inheritance overcomplicates without clarity.

3. Final Answer:

   Create classes Book and Member with clear attributes and a borrowing relationship using aggregation -> Option D

4. Quick Check:

   Clear classes + simple relationships = LLD [OK]

Hint: Clear classes + simple relationships = good LLD design [OK]

Common Mistakes:

• Overcomplicating with inheritance
• Ignoring class separation
• Skipping diagrams and explanations