Walmart Labs·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Walmart Labs data science interview with a string manipulation coding question. Pretty straightforward session, they cared more about whether you could talk through complexity and edge cases than just getting the answer.

Questions Asked (1)

Given a string, return the index of the first character that doesn't repeat anywhere else in the string. Return -1 if none exists.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Two passes: count frequencies first, then scan again for the first count-of-one character.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem: define 'first' as the earliest index in the original string, and confirm whether the string contains only ASCII or Unicode characters. Then propose a two-pass solution using a hash map to count frequencies, followed by a second pass to find the first character with count 1. Discuss trade-offs between time and space, and mention edge cases like empty string or all repeating characters.

Pro tip: In data science roles, interviewers value practical optimization: mention that for large strings, you can use a fixed-size array (e.g., 256 for ASCII) instead of a hash map to reduce overhead, and that this approach is O(n) time and O(1) space. Also, proactively discuss how you would test the solution with edge cases and measure performance.

1. Clarify requirements and constraints

Ask about the character set (ASCII vs Unicode), string length, and whether the string can be empty. Confirm that 'first' means the earliest index in the original string.

2. Choose data structure and algorithm

Decide between a hash map (general) or fixed-size array (ASCII) for frequency counting. Explain that a two-pass approach is optimal: first pass to count, second pass to find the first unique.

3. Walk through the algorithm

Describe step-by-step: iterate through the string to populate the frequency map, then iterate again to return the index of the first character with count 1. If none, return -1.

4. Analyze complexity and trade-offs

State time complexity O(n) and space complexity O(k) where k is the number of unique characters (or O(1) for fixed alphabet). Discuss alternatives like using an ordered dictionary or linked list for single-pass, but note they add complexity.

5. Handle edge cases and test

Mention edge cases: empty string, all repeating characters, single character, and strings with special characters. Suggest writing unit tests to verify correctness.

Key Points to Mention

Time complexity: O(n) with two passes, which is optimal for this problem.
Space complexity: O(k) where k is the number of unique characters; for ASCII, O(1) using a fixed array of size 256.
Trade-off between using a hash map (flexible for Unicode) and an array (faster for ASCII).
Edge cases: empty string, no unique character, string with all unique characters.
Alternative approaches: single-pass with a doubly linked list and hash map (O(n) time, O(k) space) but more complex.
Importance of clarifying the definition of 'first' and character encoding before coding.

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