Classic problem but I fumbled the first pass a bit.
Clarify the problem constraints (e.g., character set, case sensitivity, empty string) and then propose an efficient solution using a hash set to track seen characters, returning the first duplicate encountered. Walk through the algorithm with a simple example and analyze time and space complexity.
Pro tip: At Amazon, always discuss trade-offs: a hash set gives O(n) time but O(k) space; if the character set is small (e.g., ASCII), you can use a fixed-size boolean array for O(1) space. Mentioning this shows you consider practical constraints.
Ask about the character set (ASCII vs Unicode), case sensitivity, and what to return if no duplicate exists. This ensures you solve the correct problem.
Select a hash set for O(1) average-time lookups, or a boolean array if the character set is small and fixed. Explain why this choice is optimal.
Iterate through the string, checking if each character is already in the set. If yes, return it; otherwise, add it to the set. If no duplicate, return a sentinel (e.g., null).
State time complexity O(n) and space complexity O(k) where k is the number of unique characters. For fixed character sets, space is O(1).
Walk through edge cases: empty string, no duplicates, duplicate at start/end, and mixed case. Verify the algorithm handles them correctly.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.