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Bolt·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Bolt SWE interview with a coding question that sounds simple but has a twist once you start thinking about edge cases.

Questions Asked (1)

Q1

Implement a string sorting algorithm where digit characters are treated as their numeric values rather than their ASCII values.

Algorithms & Data Structures
Author's notes

My first instinct was to just use a comparator and call it a day, but the digit-as-number part tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Clarify the sorting criteria: digits should be compared by their numeric value (0-9) while non-digit characters are compared by their ASCII values. Design a custom comparator that maps each character to a sort key: for digits, use the numeric value; for non-digits, use the ASCII value. Then apply a stable sort using this comparator, ensuring that the relative order of equal elements is preserved.

Pro tip: Mention that you would handle edge cases like mixed strings, empty strings, and Unicode characters, and discuss the time complexity (O(n log n) for comparison-based sorting) and potential optimizations like counting sort for digits.

1. Clarify requirements and constraints

Ask whether the sort is lexicographic or based on custom rules, and confirm that digits are compared numerically while other characters use ASCII. Also check if the sort should be stable and if the input can contain non-ASCII characters.

2. Define the custom comparison key

For each character, if it's a digit ('0'-'9'), use its integer value (0-9); otherwise, use its ASCII value. This ensures digits are ordered 0 < 1 < ... < 9, and non-digits follow ASCII order.

3. Choose and implement the sorting algorithm

Use a comparison-based sort (e.g., merge sort or Timsort) with the custom comparator. Alternatively, if the alphabet is small, consider a counting sort variant. Ensure the sort is stable if required.

4. Test with edge cases

Test strings with mixed digits and letters, leading zeros, empty strings, and strings with only digits or only letters. Verify that digits are sorted numerically and non-digits by ASCII.

5. Analyze complexity and discuss optimizations

State the time complexity (O(n log n) for comparison sort) and space complexity. Mention that for large inputs, a radix sort or counting sort could be more efficient if the character set is limited.

Key Points to Mention

  • Custom comparator that maps digits to numeric values and non-digits to ASCII values.
  • Stability of the sort: if two characters have the same sort key, their original order should be preserved.
  • Time and space complexity: O(n log n) time for comparison-based sorting, O(n) space for merge sort or O(1) for in-place sorts.
  • Edge cases: empty strings, strings with only digits, only letters, mixed case, and Unicode characters.
  • Alternative approaches: counting sort or radix sort for better performance when the character set is small.
  • Language-specific considerations: in Java, use Comparator; in Python, use key function; in C++, use custom comparator.

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