Given a string s, return the length of the longest substring that contains no repeated characters. Checking every starting index and re-scanning forward works, but it forgets everything the previous start already learned about the string. The sliding windowSliding WindowMaintaining a running result over a contiguous range that grows or shrinks one element at a time, instead of recomputing the result for every range from scratch. technique keeps a hash map of each character's most recent index: grow the window by moving the right edge forward, and whenever a character repeats inside the current window, jump the left edge straight past that character's earlier occurrence — no need to shrink one step at a time. Every character is visited once by the right pointer, so the whole scan runs in O(n).