← Two Sigma Interview Insights
I started with the domain model which felt natural, Item and Slot were easy.
Start by clarifying requirements and scope, then define the core domain model with clear responsibilities and relationships. Next, design the user-facing API and implement an explicit state machine to manage the machine's lifecycle, ensuring all transitions are well-defined. Finally, discuss trade-offs and potential extensions.
Pro tip: Emphasize the importance of separating concerns: keep the state machine logic distinct from the domain model to avoid tight coupling and improve testability. Also, consider concurrency and error handling early, as vending machines operate in real-world scenarios with multiple users and hardware failures.
Ask clarifying questions to understand the expected functionality, constraints, and edge cases. Define what is in scope (e.g., payment methods, item types) and what is out of scope.
Identify key entities like Item, InventorySlot, Coin, Payment, and VendingMachine. Define their attributes, behaviors, and relationships, ensuring single responsibility and encapsulation.
Specify the public methods for interacting with the machine, such as selectItem, insertCoin, cancelTransaction, and dispenseItem. Ensure the API is intuitive and covers all user actions.
Define states (e.g., Idle, HasMoney, Dispensing, OutOfStock) and transitions triggered by events like coin insertion, item selection, or cancellation. Use a state pattern or explicit transition table.
Highlight design decisions, such as using composition over inheritance, and discuss potential improvements like adding new payment methods or handling concurrency.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Strategy pattern, pretty straightforward once you see it.
Start by clarifying requirements and constraints, then propose the Strategy pattern to encapsulate payment algorithms, explaining how it enables adding new payment methods without modifying existing code. Discuss trade-offs and consider extensions like combining with Factory or Adapter patterns for real-world flexibility.
Pro tip: Mention that payment processing often involves external services, so you'd also consider the Adapter pattern to integrate third-party APIs, and emphasize testability by mocking payment strategies.
Ask about supported payment methods (cash, card, mobile), whether new methods will be added frequently, and any constraints like offline operation or security.
Explain that hardcoding payment logic leads to rigid code; each new method requires modifying the vending machine class, violating Open/Closed Principle.
Introduce the Strategy pattern: define a PaymentStrategy interface with a pay() method, and implement concrete strategies for each payment type. The vending machine delegates to the selected strategy.
Describe how the vending machine holds a reference to a PaymentStrategy, which can be set at runtime. Optionally, use a Factory to create strategies based on user selection.
Compare with other patterns (e.g., State, Command) and mention that Adapter may be needed for third-party APIs. Highlight benefits: flexibility, testability, and adherence to SOLID principles.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This is where the interview got uncomfortable.
Start by clarifying the vending machine's API and state model, then outline a test plan that maps each requirement to specific test cases. For each case, define the setup, action, and expected outcome, and discuss how to handle concurrency and edge conditions. Emphasize test isolation and deterministic behavior.
Pro tip: Use parameterized tests for happy path and exact change scenarios to reduce duplication, and mock time or randomness to make tests deterministic. Also, consider property-based testing for invariants like 'balance never negative'.
Ask clarifying questions about the vending machine's interface, state, and expected behaviors. Define the public methods (e.g., selectItem, insertCoin, cancel) and the internal state (e.g., inventory, balance).
For each required scenario (happy path, exact change, change due, insufficient funds, sold-out, cancel/refund, concurrent purchases), outline specific test cases with inputs and expected outputs. Consider edge cases like multiple coins, invalid selections, and race conditions.
Create reusable setup code to initialize the vending machine with known inventory and coin denominations. Use mocks or stubs for external dependencies (e.g., coin dispenser, inventory sensor) to isolate the unit under test.
Write tests using a framework like JUnit or pytest, ensuring each test is independent and deterministic. Use assertions to verify state changes, return values, and side effects (e.g., coins dispensed).
For concurrent purchases, simulate multiple threads or use a concurrency testing framework to verify thread safety. Discuss strategies like locks, atomic operations, or transactional behavior, and test for race conditions.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.