← Capital One Interview Insights
Start by clarifying the problem: we need to count all length-3 substrings where the first and last characters match case-insensitively. Then, walk through a simple O(n) solution that iterates through the string once, comparing characters at positions i and i+2 after normalizing case, and incrementing a counter when they match.
Pro tip: Mention that you can avoid creating substrings or using extra space by comparing characters directly, which shows attention to efficiency and clean coding. Also, explicitly handle edge cases like strings shorter than 3 characters.
Confirm that substrings are contiguous, length exactly 3, and that case-insensitivity means 'A' and 'a' are considered equal. Ask if the input can be empty or have non-alphabetic characters.
Describe generating all length-3 substrings and checking each one, noting it would be O(n) time and O(1) extra space if done without creating new strings, but O(n) substrings if created.
Explain that you can iterate from index 0 to n-3, and for each i, compare the characters at i and i+2 after converting both to lowercase (or using a case-insensitive comparison). Increment a counter if they match.
State that the optimized solution runs in O(n) time and O(1) extra space, since it only uses a counter and a few variables.
Walk through a sample string like 'AbcA' to verify the count, and mention edge cases such as strings of length less than 3 (return 0) and strings with all matching characters.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.