Boss: The "abba" Trap

Boss level — two bugs. The longest-unique-substring service is wrong for "abba" and for every string.

longestUnique(s) returns the length of the longest substring without repeated characters.

Fix the code so every test passes. Keep the function name longestUnique.

Test cases

• longestUnique("abcabcbb") → 3
• longestUnique("bbbbb") → 1
• longestUnique("pwwkew") → 3
• longestUnique("abba") → 2
• longestUnique("") → 0

Stuck? Hints go from a nudge to the exact fix.