Capital One·Machine Learning Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Capital One ML engineer interview, one coding question that looked deceptively simple on the surface. The algorithm was described in plain English rather than given as a standard problem, which threw me off more than I expected.

Questions Asked (1)

Implement an algorithm from a verbal description: given a non-negative integer array, initialize a result variable to 0. Find the first non-negative number x from the left, then compare subsequent elements to x. If a subsequent element is smaller, add x to the result and restart from step 1. Otherwise, subtract x from that element. Return the result when the array is exhausted.

Algorithms & Data Structures

Author's notes

The description sounds clear when you read it back but in the moment I kept second-guessing what 'subtract x' meant for the array state and whether I was mutating in place or working on a copy.

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AI HintsAI Generated

Suggested Approach

First, restate the algorithm in your own words and walk through a small example to confirm understanding. Then, outline a step-by-step implementation using a loop and a pointer, handling edge cases like all zeros or negative numbers. Finally, discuss time and space complexity and potential optimizations.

Pro tip: Clarify ambiguities upfront: ask whether the array can be modified in place and whether 'non-negative' includes zero. This shows attention to detail and prevents incorrect assumptions.

1. Clarify and Confirm

Ask clarifying questions about edge cases (e.g., empty array, all zeros, negative numbers) and confirm the expected behavior. Restate the algorithm to ensure alignment.

2. Walk Through an Example

Choose a small array (e.g., [3, 1, 4, 2]) and manually trace the algorithm to verify your understanding and identify any pitfalls.

3. Design the Algorithm

Outline the steps: iterate through the array, maintain a pointer to the current x, and update the result and array elements as described. Consider using a while loop with an index.

4. Analyze Complexity

Determine the time complexity (likely O(n) or O(n^2) depending on implementation) and space complexity (O(1) if in-place). Discuss trade-offs.

5. Test and Optimize

Test with edge cases (empty array, single element, all zeros, negative numbers). If time permits, suggest optimizations or alternative approaches.

Key Points to Mention

Handling of zero as a non-negative number and its implications.
In-place modification versus creating a new array.
Time and space complexity analysis.
Edge cases: empty array, all zeros, negative numbers, and large inputs.
Potential for integer overflow if values are large.
Clarifying the restart condition: after adding x to result, restart from the beginning of the remaining array.

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