Knew immediately it was a sliding window problem, which felt good.
Use a sliding window with two pointers to maintain a window of unique characters, expanding the right pointer and shrinking the left when a duplicate is found. Track the maximum window length seen. This yields O(n) time and O(min(n, alphabet)) space.
Pro tip: Clarify assumptions upfront (e.g., ASCII vs Unicode, empty string, case sensitivity) and discuss trade-offs between the optimal sliding window and simpler brute-force approaches to show engineering maturity.
Ask about character set (ASCII/Unicode), case sensitivity, and expected input size. Confirm behavior for empty strings and strings with all unique characters.
Mention that checking all substrings for uniqueness takes O(n^3) or O(n^2) with a set, establishing a baseline before optimizing.
Use two pointers (left, right) and a hash map/set to track characters in the current window. Expand right, and when a duplicate is found, move left past the previous occurrence.
Explain that each character is visited at most twice, giving O(n) time. Space is O(min(n, alphabet size)). Optionally, use an array for fixed ASCII to improve constant factors.
Walk through examples like 'abcabcbb' (3), 'bbbbb' (1), 'pwwkew' (3), and empty string (0) to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.