Capital One·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Capital One SWE interview that came down to a single OOP design problem. Pretty straightforward if you've seen this kind of thing before, but there are a few edge cases that'll trip you up if you're not careful.

Questions Asked (1)

Design and implement a simple bank system class. The constructor takes a 1-indexed array of initial balances. You need to support deposit, withdraw, and transfer operations, where each must validate the account number is in range and any debit has sufficient funds, returning false without modifying state if validation fails.

Algorithms & Data StructuresSystem Design

Author's notes

The core logic isn't hard but I almost forgot the 1-indexed part and was about to write a bounds check that was off by one.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and edge cases, then design a class that stores balances in a 1-indexed array (or adjust indices) and implements each operation with validation checks. For each operation, validate the account number and sufficient funds before mutating state, returning false immediately if any check fails. Finally, discuss time and space complexity and potential concurrency considerations.

Pro tip: Mention that you would use a 1-indexed array by allocating size n+1 and ignoring index 0, which simplifies index validation and matches the problem statement. Also, emphasize that validation must happen before any state change to ensure atomicity.

1. Clarify requirements and edge cases

Ask about input constraints, expected return values, and whether operations should be thread-safe. Confirm that account numbers are 1-indexed and that invalid operations should return false without side effects.

2. Design the data structure

Choose an array to store balances, using a 1-indexed approach (e.g., array of size n+1). Discuss why this is efficient for O(1) access and updates.

3. Implement validation logic

For each operation, check that the account number is within [1, n] and that any debit (withdraw or transfer source) has sufficient funds. Return false immediately if validation fails.

4. Implement operations

Write deposit, withdraw, and transfer methods that perform the validated updates. For transfer, validate both accounts and sufficient funds before modifying either balance.

5. Analyze complexity and discuss improvements

State that all operations are O(1) time and O(n) space. Mention potential concurrency issues and how to address them (e.g., using locks) if needed.

Key Points to Mention

1-indexed array implementation (size n+1, ignore index 0)
Validation order: check account number range first, then sufficient funds
Atomicity: no state changes if validation fails
Return false on invalid operations, true on success
Time complexity O(1) per operation, space O(n)
Concurrency considerations (e.g., thread safety) if relevant

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