Capital One·Software Engineer·Onsite - System Design / Architecture·Intermediate

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May 2026

Summary

Capital One SWE interview that was basically one big system design problem: build a banking system from scratch with account creation, deposits, transfers, and some kind of leaderboard for most active accounts. Input came through CSV or JSON parsing which added a layer I didn't fully expect.

Questions Asked (1)

Design a banking system that supports account creation, deposits, transfers between accounts, and a way to retrieve the most active accounts ranked by total monetary activity. Commands are issued by parsing structured input like CSV or JSON.

System DesignData ModelingAlgorithms & Data Structures

Author's notes

The parsing requirement threw me a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a modular system with clear separation between command parsing, core banking operations, and activity ranking. Use appropriate data structures (e.g., hash maps for accounts, balanced trees or heaps for ranking) and discuss trade-offs for scalability and consistency.

Pro tip: Emphasize idempotency and error handling for commands, as banking systems must handle duplicate or malformed inputs gracefully. Also, mention how you would extend the design to support concurrency and distributed deployment.

1. Clarify Requirements and Constraints

Ask about expected scale (number of accounts, transactions per second), consistency requirements, and input format specifics. Confirm whether ranking should be real-time or batch, and how ties are broken.

2. Design Core Data Model and Operations

Define Account and Transaction entities, and specify operations: create account, deposit, transfer (with atomicity). Use a hash map for account lookup and ensure transfers are atomic to maintain consistency.

3. Implement Command Parsing and Validation

Design a parser for CSV/JSON that validates input, handles errors, and maps commands to operations. Ensure idempotency by using unique command IDs or deduplication.

4. Design Activity Ranking System

Track total monetary activity per account (sum of deposits and transfers). Use a data structure that supports efficient updates and retrieval of top-K accounts, such as a balanced BST or a heap with lazy updates.

5. Address Scalability, Consistency, and Extensions

Discuss partitioning, replication, and concurrency control (e.g., locks or optimistic concurrency). Mention how to handle failures and ensure data durability.

Key Points to Mention

Use of hash maps for O(1) account lookup and balanced trees/heaps for O(log n) ranking updates.
Atomicity and isolation for transfers to prevent partial updates and ensure consistency.
Idempotency of commands to handle retries and duplicate submissions.
Efficient top-K retrieval using a min-heap of size K or a balanced BST with subtree sizes.
Scalability considerations: sharding by account ID, read replicas for ranking queries.
Error handling and validation for malformed input, insufficient funds, and non-existent accounts.

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