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Longest Substring Without Repeating Characters is a medium coding interview question covering strings, sliding window, hash maps — the kind asked at FAANG (Facebook/Meta, Apple, Amazon, Netflix, Google) and AI-first companies like Anthropic, OpenAI, and Nvidia.
Given a string s, find the length of the longest substring without repeating characters. Example: "abcabcbb" -> 3 (the substring "abc").
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