Return the Longest Substring Without Repeating Characters

Given a string s, find and return the longest contiguous substring that contains no repeated characters. Return the substring itself rather than its length. If multiple longest valid substrings exist, return the leftmost one (the one with the smallest starting index). The string may contain letters, digits, spaces, and other ASCII characters. Examples Input: s = "abcabcbb" Output: "abc" Explanation: "abc" is the longest substring without repeating characters. Input: s = "pwwkew" Output: "wke" Explanation: "wke" is the longest substring without repeating characters; "pwke" is not contiguous. Constraints 0 <= len(s) <= 10^5 Target time complexity: O(n) Target extra space complexity: O(min(n, alphabet size)) Example Input abcabcbb Output abc