Find the Longest Substring without Repeated Characters

Given a string, find the maximum length of a contiguous substring containing no repeated character. ### Constraints and Clarifications Use a length-only return value as the practice interface, so tied longest substrings need no tie-breaking rule. Treat the input as a sequence of characters with exact equality and no normalization; no encoding or numerical size bounds were reported. The empty string returns zero. ### Example For "abcaef", the longest valid substring is "bcaef", so the length is 5. ### What a Strong Answer Covers Maintain a valid sliding window, advance its left boundary only when necessary, and avoid moving it backward when a character was last seen before the current window. ### Follow-up Questions - Why is max(left,last_seen+1) needed in a last-position implementation? - How would you return a substring as well as its length after defining a tie rule?