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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'm having the same issue
Ok thanks My Bad
You're reading it wrong. But it was not described as clearly as possible.
You have to read the operation first, then the array, which is the resultant array from that operation.
I guess there is a typo in the description in the first test case, the last element in the initial array should be 21 and not 12 for it to work the way it's written.