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Robinhood·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jul 2026

Summary

Robinhood SWE interview with a coding round that was basically a mini system design disguised as an OOP problem. You're building an in-memory payment system from scratch with friend requests and money transfers, and the devil is in the edge cases.

Questions Asked (1)

Q1

Design and implement an in-memory payment system that supports user registration, friend requests (with a separate accept step referencing the original request by ID), and money transfers that only work between friends with sufficient balance.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

The friend request indirection tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the core entities (User, FriendRequest, Transaction) and their relationships. Then design the data structures and algorithms for each operation, focusing on efficiency and correctness. Finally, discuss trade-offs, edge cases, and potential extensions like concurrency and persistence.

Pro tip: Emphasize idempotency and atomicity in money transfers to prevent double-spending and ensure consistency, especially under concurrent requests. Also, consider using a graph representation for friendships to efficiently check friend status.

1. Clarify Requirements and Scope

Ask questions to understand constraints: expected scale, concurrency needs, persistence requirements, and whether friend requests are bidirectional. Define success criteria and assumptions.

2. Design Data Model and Structures

Outline classes/structures for User (with balance), FriendRequest (with status and IDs), and friendships (e.g., adjacency list or set). Choose appropriate data structures for fast lookup and updates.

3. Implement Core Operations

Detail algorithms for user registration, sending/accepting friend requests (using request ID), and money transfer with friend check and balance validation. Ensure operations are atomic and handle edge cases.

4. Address Concurrency and Consistency

Discuss locking, transactions, or optimistic concurrency to prevent race conditions in balance updates and friend request acceptance. Mention idempotency for transfers.

5. Analyze Trade-offs and Extensions

Compare design choices (e.g., in-memory vs. persistent, locking granularity) and suggest improvements like caching, sharding, or event sourcing for scalability.

Key Points to Mention

  • Use of unique IDs for users and friend requests to reference them unambiguously.
  • Data structures: hash maps for users and friend requests, sets or adjacency lists for friendships.
  • Atomicity in money transfer: check friend status, verify balance, and update balances in a single transaction or with locks.
  • Idempotency of transfer requests to avoid duplicate processing.
  • Handling of edge cases: insufficient balance, non-friend transfers, invalid request IDs, and concurrent friend request acceptance.
  • Trade-offs between simplicity and scalability, e.g., in-memory vs. distributed, and potential use of databases for persistence.

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