I believe this is optional since:

1. I attached the chorus:

    Never gonna give you up
    Never gonna let you down
    Never gonna run around and desert you
    Never gonna make you cry
    Never gonna say goodbye
    Never gonna tell a lie and hurt you 

2. The first example test case is a check for support of all strings with indices from 0 to 5 inclusive.

Thanks for your suggestion.

You're not supposed to sort the input.