Ziphq·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

ZipHQ software engineer interview with a vending machine OOP design problem. Pretty involved for a single coding round, they wanted full implementation plus tests, not just a sketch.

Questions Asked (1)

Design and implement the core logic for a vending machine using object-oriented principles. The machine should store products with ids, prices, and quantities; accept a fixed set of bill denominations; support inserting money, selecting a product, dispensing change, and canceling a transaction. Handle edge cases like insufficient funds, out-of-stock items, and inability to make exact change.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

The change-dispensing part is what got me.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and assumptions, then outline the core classes and their responsibilities using OOP principles. Walk through the main flows (insert money, select product, dispense, cancel) and discuss how you handle edge cases like insufficient funds, out-of-stock, and change-making. Emphasize clean separation of concerns, extensibility, and testability.

Pro tip: Proactively discuss how you would make the design extensible (e.g., adding new payment methods or product types) and how you would test critical edge cases like exact change scenarios. This shows foresight and engineering maturity.

1. Clarify Requirements and Assumptions

Ask clarifying questions about product inventory, accepted bill denominations, change-making rules, and concurrency. State your assumptions explicitly to scope the problem.

2. Identify Core Objects and Responsibilities

Define classes like VendingMachine, Product, Inventory, CashRegister, and Transaction. Assign clear responsibilities to each, following single responsibility and encapsulation.

3. Design Key Interactions and State Management

Outline how money insertion, product selection, dispensing, and cancellation work. Describe the state machine (e.g., idle, has money, dispensing) and how transitions occur.

4. Handle Edge Cases and Error Conditions

Explain strategies for insufficient funds, out-of-stock items, and inability to make exact change. Discuss how to maintain cash inventory and prioritize denominations for change.

5. Discuss Extensibility and Testing

Mention how to extend the design (e.g., new payment methods, dynamic pricing) and how to test critical paths and edge cases with unit tests.

Key Points to Mention

Encapsulation of product and cash inventory with clear interfaces
State pattern or state machine for transaction flow
Change-making algorithm (e.g., greedy with fallback or dynamic programming)
Handling concurrency (e.g., locking or atomic operations) if multiple users
Use of exceptions or result objects for error handling
Separation of concerns to allow easy addition of new payment methods or product types

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.