Google·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Google SWE coding round with a string manipulation problem that looked straightforward but had enough edge cases to keep you honest. The problem involved chunking a string and building a validation result using checksum-based character selection.

Questions Asked (1)

Given a string, a validation string, and an integer k, split the original string into fixed-length chunks of size k. For each chunk, compute a checksum from the character values, then use that checksum to select a character from the validation string. Concatenate the selected characters in order and return the result. Handle the case where the string length isn't evenly divisible by k.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The core logic isn't bad once you break it down, chunk by chunk, compute sum, map to index via modulo.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First, clarify all requirements and edge cases, especially how to handle the final chunk if the string length is not a multiple of k. Then, outline a step-by-step algorithm: iterate over the string in chunks of size k, compute the checksum for each chunk, use modulo to map the checksum to a valid index in the validation string, and concatenate the selected characters. Finally, discuss time and space complexity and potential optimizations.

Pro tip: Explicitly ask the interviewer how to handle the last chunk if it's shorter than k—whether to pad it, ignore it, or process it as-is—because this ambiguity is often the key differentiator in candidate performance.

1. Clarify Requirements and Edge Cases

Ask about the checksum definition (e.g., sum of ASCII values), how to handle the last chunk if length % k != 0, and whether the validation string can be empty. Confirm input types and expected output.

2. Design the Algorithm

Iterate through the string in steps of k. For each chunk, compute the checksum (e.g., sum of character codes). Use checksum % validationString.length to select a character. Append to result.

3. Handle the Final Chunk

If the string length is not divisible by k, decide with the interviewer whether to process the remaining characters as a shorter chunk, pad with a default character, or ignore it. Implement accordingly.

4. Analyze Complexity and Optimize

State time complexity O(n) and space O(n/k) for the output. Discuss potential optimizations like early termination or using a rolling checksum if applicable.

5. Test with Examples

Walk through a few test cases: exact multiple, remainder, empty string, k=1, k > string length, and validation string of length 1. Verify correctness and edge case handling.

Key Points to Mention

Definition of checksum: sum of ASCII/Unicode values of characters in the chunk.
Modulo operation to map checksum to a valid index in the validation string.
Handling of the last chunk when string length is not a multiple of k (e.g., process as-is, pad, or ignore).
Time and space complexity analysis: O(n) time, O(n/k) space for output.
Edge cases: empty string, k=0 or negative, validation string empty, k > string length.
Potential optimizations: avoid unnecessary string concatenation by using a list/array and joining at the end.

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