Given a string `s`, return the LENGTH of the longest substring that contains no repeating characters. A substring is a contiguous run of characters (no gaps).
Example: s = "abcabcbb" → 3 ("abc")
Brute force: For every start index, extend right until a character repeats. (time O(n²), space O(n))
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Why the best approach wins
Brute force re-scans every possible start from scratch — O(n²). The sliding window never looks back: L and R each only move right, and remembering the last index of each character lets L leap straight past a repeat instead of inching forward. That turns the whole thing into one O(n) pass.
Brute force: O(n²) time / O(n) spaceSliding window: O(n) time / O(k) space
Your turn — implement lengthOfLongestSubstring
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