Longest Substring Without Repeats

Return the longest substring of s with no repeated characters. A substring is contiguous, unlike a subsequence.

If several substrings tie for longest, return the one that appears first. Your solution should run in $O(n)$ time: one pass with a sliding window, not checking every substring.

Examples

longest_unique_substring("abcabcbb")   # "abc"
longest_unique_substring("pwwkew")     # "wke"
longest_unique_substring("")           # ""