Robinhood·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Robinhood software engineer round, basically a mini system design plus coding problem rolled into one. You're building a social payment system from scratch with validation logic and clean class structure. Not a leetcode grind, more like a design-your-own-bank-with-friends problem.

Questions Asked (1)

Design and implement a small social payment system that processes a list of string-based commands (user sign-up, friend requests, accepting friend requests, sending money, and deposits). Each command has its own validation rules. Output each user's final balance after all events are processed.

System DesignAlgorithms & Data StructuresData Modeling

Author's notes

This one is more involved than it looks.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design a clean data model with users, friendships, and balances. Implement command parsing and validation in a modular way, ensuring atomicity and correct ordering of operations.

Pro tip: Emphasize idempotency and error handling: commands may be repeated or invalid, and the system should gracefully reject them without corrupting state. Also, discuss how you would scale the design if the command list were huge.

1. Clarify Requirements and Edge Cases

Ask questions to understand command formats, validation rules, and expected outputs. Identify edge cases like duplicate users, self-friending, insufficient balance, and concurrent operations.

2. Design Data Model

Define core entities: User (with balance), Friendship (pending/accepted), and Transaction. Choose appropriate data structures (e.g., hash maps for users, adjacency sets for friendships) to support efficient lookups and updates.

3. Implement Command Processing

Parse each command string, validate according to rules, and update the data model atomically. Use a dispatcher pattern or switch-case to handle each command type, ensuring errors are caught and reported without affecting other operations.

4. Handle Output and Final Balances

After processing all commands, iterate through users to output their final balances in a specified order. Consider sorting or formatting requirements.

5. Test and Validate

Walk through example scenarios, including invalid commands, to verify correctness. Discuss potential optimizations and scalability if needed.

Key Points to Mention

Data structures: hash maps for users, sets for friendships, and careful handling of pending vs. accepted requests.
Validation rules: unique usernames, no self-friending, only friends can send money, sufficient balance, positive amounts.
Atomicity: ensure each command either fully succeeds or fails without partial state changes.
Idempotency: handle duplicate commands gracefully, especially for deposits and friend requests.
Error handling: log or ignore invalid commands without crashing, and continue processing subsequent commands.
Scalability: discuss how to handle large command lists, potential concurrency issues, and persistence if needed.

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