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Boston Consulting Group·AI Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

BCG AI Engineer interview with a coding question around string processing. Pretty straightforward on the surface but easy to overthink.

Questions Asked (1)

Q1

Given a string of letters, count how many times consecutive characters differ when treated case-insensitively.

Algorithms & Data Structures
Author's notes

Took me a second to realize they just wanted adjacent comparisons, not anything fancy.

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AI HintsAI Generated

Suggested Approach

Clarify the problem by restating it: count adjacent pairs where the lowercase versions of the characters differ. Then walk through a simple linear scan, comparing each character to the next after normalizing case, and discuss edge cases like empty strings or single characters.

Pro tip: Mention that you can avoid repeated case conversions by normalizing the string once upfront, and note that this is a classic O(n) time, O(1) space problem—showing you think about efficiency even for simple tasks.

1. Clarify the problem

Restate the question to ensure you understand: count adjacent character pairs where the characters differ when both are converted to the same case. Ask about edge cases like empty string, single character, or non-letter characters.

2. Outline the approach

Explain that you will iterate through the string once, comparing each character with the next after normalizing case (e.g., using lower() or upper()). Keep a counter for differences.

3. Walk through an example

Choose a small example like 'aAbB' and manually show how the count is computed: compare 'a' vs 'A' (same), 'A' vs 'b' (different), 'b' vs 'B' (same) → count = 1. This demonstrates your logic.

4. Discuss complexity and edge cases

State that the solution runs in O(n) time and O(1) extra space (if normalizing on the fly) or O(n) if creating a normalized copy. Mention handling of empty string (return 0) and single character (return 0).

5. Write pseudocode or code

If asked, provide clean pseudocode or code in a language of your choice, ensuring it handles case insensitivity correctly and is efficient.

Key Points to Mention

  • Case-insensitive comparison: convert both characters to the same case before comparing.
  • Linear scan: iterate through the string once, comparing each character with the next.
  • Edge cases: empty string, single character, strings with non-letter characters (if allowed).
  • Time complexity: O(n) where n is the length of the string.
  • Space complexity: O(1) if normalizing on the fly, O(n) if creating a normalized copy.
  • Potential pitfalls: off-by-one errors when accessing the next character, and ensuring the loop stops at the second-to-last character.

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