Many candidates focus purely on High-Level Design (HLD) and LeetCode, entirely neglecting Low-Level Design (LLD). But for SDE-1, SDE-2, and Senior Software Engineer roles, companies like Amazon, Uber, and Microsoft will thoroughly evaluate your ability to write modular, extensible, and clean object-oriented code.

You don't need to read the entire Gang of Four book. You just need to know the core patterns and principles. Here is the ultimate 7-day preparation roadmap.

The 7-Day LLD Countdown Plan

Day 1: Warming Up (SOLID Principles)

Before touching design patterns, you must understand the rules that govern them. SOLID is the bedrock of OOD (Object Oriented Design).

  • S - Single Responsibility: A class should have one and only one reason to change.
  • O - Open/Closed: Objects or entities should be open for extension but closed for modification.
  • L - Liskov Substitution: Objects of a superclass shall be replaceable with objects of its subclasses without breaking the application.
  • I - Interface Segregation: A client should never be forced to implement an interface that it doesn't use.
  • D - Dependency Inversion: Depend upon abstractions, not concretions.

Day 2: Strategy Pattern

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Common Interview Usage: Pricing calculators, sorting algorithms, authentication methods, or payment processing (e.g., Credit Card vs. PayPal).

Day 3: Observer Pattern

The Observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Common Interview Usage: Designing a notification system, event-driven architectures, stock market tickers, or chat systems.

Day 4: Factory and Singleton Patterns

These two creational patterns are incredibly common.

  • Factory Method: Defines an interface for creating an object, but lets subclasses decide which class to instantiate. Great for when you don't know beforehand the exact types and dependencies of the objects your code should work with.
  • Singleton: Ensures a class has only one instance and provides a global point of access to it. (Be prepared to discuss Thread-safe Singletons and why Singletons can sometimes be considered an anti-pattern!).

Day 5: Flyweight Pattern

The Flyweight pattern is a structural design pattern that lets you fit more objects into the available amount of RAM by sharing common parts of state between multiple objects instead of keeping all of the data in each object.

Common Interview Usage: Designing a text editor (sharing character formatting), or rendering millions of trees in a game (where the mesh/texture is shared, and only the coordinates vary).

Day 6: State Pattern

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Common Interview Usage: Designing a Vending Machine, an Elevator, or a document approval workflow. Instead of massive `if-else` or `switch` statements, state transitions are handled cleanly via polymorphic state classes.

Day 7: Multi-threading & Concurrency

For mid-to-senior roles, writing a thread-safe LLD is critical. Spend your last day reviewing concurrency concepts.

  • Thread-safety: Mutexes, Locks, and Semaphores.
  • Concurrency in DBs: Understand Optimistic vs. Pessimistic Locking.
  • Isolation Levels: Know the difference between Read Uncommitted, Read Committed, Repeatable Read, and Serializable, and the anomalies they prevent (Dirty reads, Non-repeatable reads, Phantom reads).

The Practitioner's Edge: What Actually Differentiates You

Generic candidates start writing code immediately and cram everything into a single `main` class. Strong candidates define interfaces first. They think about how the system will change in the future (e.g., "What if we add a new payment method?") and apply the right pattern (Strategy) to ensure the core logic remains untouched.

Remember: your code must be executable, clean, and extensible. Name your variables well, create distinct classes, and always write modular code.

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