Longest Substring Without Repeating Characters

Given a string, return the length of its longest substring containing no repeated characters. The exact interview signature, character set, constraints, and examples remain unspecified. Requirements Given a string s, return the length of the longest substring containing no repeated characters. The chosen substring must be contiguous, and every character inside it must be distinct. The exact function signature, character set, constraints, and empty-input contract are unspecified; clarify them before coding. Notes Scan from left to right while storing each character's latest index. When a character repeats inside the active range, move the left boundary to one position after its previous index; never move the left boundary backward. Update the latest index and the best length after restoring the no-duplicate invariant. A count-map window is also valid, but the latest-index form avoids repeated single-step shrinking. The canonical scan runs in O(n) time and O(min(n, |alphabet|)) auxiliary space. Preparation Implement both the latest-index and count-map versions from a blank editor, then explain the invariant each version maintains. Dry-run empty input, one repeated character, a repeat outside the active range, and a repeat of the current leftmost character. Verify that the left boundary uses max(left, previous_index + 1) so an old occurrence cannot move the window backward.