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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.
I agree with you, if he adds a negative number to the array, the code will give an erroneous value
the instructions say that "The numbers will also all be unique"
so you'll never get two of the same values, e.g. two '3's in your example