The base problem is easy enough, I've done anagram checks before.
First, clarify the constraints: the strings are likely ASCII or lowercase English letters, and O(1) extra space means a fixed-size array (e.g., 256 or 26 integers) is acceptable. Then, propose a frequency count approach: iterate through both strings, incrementing counts for the first and decrementing for the second, and finally check that all counts are zero. Emphasize that this runs in O(n) time and uses O(1) space because the array size is constant.
Pro tip: Mention that you would confirm the character set with the interviewer, as the fixed array size depends on it (e.g., 26 for lowercase letters, 256 for ASCII). This shows attention to detail and avoids incorrect assumptions.
Ask about the character set (e.g., ASCII, Unicode) and whether the strings can contain spaces or punctuation. Confirm that O(1) extra space allows a fixed-size array.
If the strings have different lengths, they cannot be anagrams, so return false immediately. This is a quick O(1) check.
Create an integer array of size equal to the number of possible characters (e.g., 256 for ASCII). Initialize all counts to zero.
Iterate through the first string, incrementing the count for each character. Then iterate through the second string, decrementing the count for each character.
After processing both strings, check that every element in the frequency array is zero. If any is non-zero, return false; otherwise, return true.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.