Capital One·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Capital One SWE interview with a straightforward coding problem around bank account operations. Nothing too wild, but the implementation details trip you up if you're not careful about edge cases.

Questions Asked (1)

Design and implement a Bank class that supports transfer, deposit, and withdraw operations on n accounts with initial balances. Each method should validate account numbers and balances before proceeding, and return a boolean indicating success or failure. Account numbers are 1-indexed.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Seemed simple at first and I basically started coding before fully thinking through the indexing.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design a class with an array of balances (1-indexed) and methods that validate inputs before mutating state. Implement each operation with O(1) time complexity, ensuring atomicity for transfers and returning booleans for success/failure.

Pro tip: Mention that you would use a lock or synchronized methods to ensure thread safety, especially for transfers, and discuss how you would handle concurrent access to prevent race conditions.

1. Clarify requirements and edge cases

Ask about constraints: number of accounts, initial balance range, whether negative balances are allowed, and if operations need to be thread-safe. Confirm that account numbers are 1-indexed and that methods return boolean.

2. Design the class structure

Use an array (or list) of size n+1 to store balances, ignoring index 0. Define methods: deposit(account, amount), withdraw(account, amount), transfer(from, to, amount). Each method validates account numbers and amounts before proceeding.

3. Implement validation and operations

For each method, check that account numbers are within 1..n and amounts are positive. For withdraw and transfer, ensure sufficient balance. Perform the operation atomically and return true on success, false otherwise.

4. Analyze complexity and trade-offs

All operations are O(1) time and O(n) space. Discuss potential improvements like using a hash map for sparse accounts or adding thread safety with locks, weighing simplicity vs. concurrency.

5. Test with edge cases

Mention testing invalid account numbers, zero/negative amounts, insufficient funds, and concurrent transfers. Ensure methods return false appropriately without altering state.

Key Points to Mention

1-indexed array with size n+1 to simplify account number mapping.
Input validation: account number bounds, positive amount, sufficient balance.
Atomicity of transfer: ensure both debit and credit happen or neither.
Time complexity O(1) per operation, space O(n).
Thread safety considerations: synchronized methods or locks for concurrent access.
Return boolean to indicate success/failure without throwing exceptions.

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