Coinbase·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Coinbase Data Scientist interview with a coding question that looks trivial until you hit the all-nines edge case. Pretty standard technical screen vibe, though the constraint about not converting the array to a big integer directly adds a bit of wrinkle.

Questions Asked (1)

You're given an array of digits representing a non-negative integer (most significant digit first). Add one to that integer and return the result as an array. You can't just convert the whole thing to a big integer directly. What's your approach and what are the time and space complexities?

Algorithms & Data Structures

Author's notes

The basic case is easy, just increment the last element.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then propose a linear scan from the least significant digit, handling carries and the all-9s case. Explain the time and space complexity, emphasizing O(n) time and O(1) extra space (or O(n) if a new array is needed).

Pro tip: Mention that you'd discuss trade-offs between modifying in-place versus creating a new array, and how this relates to real-world data processing where immutability might be preferred.

1. Clarify requirements and edge cases

Ask about input size, whether the array can be modified in-place, and confirm handling of leading zeros (e.g., [0] -> [1], [9,9] -> [1,0,0]).

2. Outline the algorithm

Traverse from the end, add 1 to the last digit, and propagate carry leftwards. If a carry remains after the first digit, prepend 1.

3. Analyze complexity

Time complexity is O(n) in the worst case (all 9s). Space complexity is O(1) extra if modifying in-place, or O(n) if creating a new array.

4. Discuss implementation details

Explain how to handle the carry with a loop or recursion, and how to insert at the beginning efficiently (e.g., using a new array or list insertion).

5. Test with examples

Walk through examples like [1,2,3] -> [1,2,4], [9,9,9] -> [1,0,0,0], and [0] -> [1] to validate the approach.

Key Points to Mention

Time complexity O(n) and space complexity O(1) extra space (or O(n) if new array).
Handling of carry propagation and the all-9s edge case.
In-place modification versus creating a new array and trade-offs.
Avoiding integer conversion due to potential overflow.
Potential for early termination if no carry occurs.
Relevance to data science: efficient array manipulation and handling large datasets.

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