Start by clarifying the problem and edge cases, then propose a two-pass solution using a hash map to count character frequencies. In the first pass, build the frequency map; in the second pass, iterate through the string and return the index of the first character with a count of 1. If no such character exists, return -1.
Pro tip: Mention that you can optimize space by using an array of size 256 if the character set is ASCII, and discuss the trade-off between time and space. Also, explicitly state the time and space complexity of your solution to demonstrate thoroughness.
Ask about the character set (e.g., ASCII or Unicode), string length limits, and whether the string can be empty. Confirm that 'first' means the smallest index.
Select a hash map (or an array if the character set is small) to store character frequencies. Explain why this allows O(1) average-time lookups.
Iterate through the string once, updating the frequency count for each character in the map.
Iterate through the string again, and return the index of the first character whose frequency is 1. If none, return -1.
State that time complexity is O(n) and space complexity is O(k) where k is the number of distinct characters. Walk through a few test cases, including empty string and all repeating characters.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.