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I nailed the sliding window version, complexity and all.
First, solve the problem using the standard sliding window approach, then propose an alternative that uses a hash map to store the last seen index of each character and a 'start' variable to track the beginning of the current substring, updating it when a repeat is found. Emphasize that this method avoids moving a left pointer explicitly by jumping the start to the last occurrence + 1.
Pro tip: Show awareness of trade-offs: the hash map approach is still O(n) but may use more memory; mention that the sliding window is more intuitive but the alternative demonstrates deeper understanding of the problem's invariants.
Restate the problem to ensure understanding: find the length of the longest substring without repeating characters. Ask about character set (ASCII vs Unicode) and edge cases (empty string, all unique).
Briefly explain the sliding window approach with two pointers and a set, noting its O(n) time and space complexity. This establishes a baseline.
Describe a method using a hash map to store the last seen index of each character and a 'start' variable. When a repeat is found, update 'start' to max(start, last_seen[char] + 1) and update the max length.
Trace the algorithm on a sample string like 'abcabcbb' to illustrate how 'start' jumps and how the max length is computed.
State that the alternative is also O(n) time and O(min(n, alphabet)) space. Compare with sliding window: both are efficient, but the hash map approach may be less intuitive yet avoids explicit left pointer movement.
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