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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Kata stipulates "an array with exactly 5 strings". Not great for general use, but meets the requirements of the kata.
there's sure to be a logical reason for that
It is not stated anywhere in the description that you can't reuse letters. If you look at the author's solution, his solution would also "fail" the test you put.