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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.
Not an issue, this is covered in the description:
Why is this an issue? Description clearly explaino what to return in such cases. Also inyour example, 0, 1, 2, 4, 5, and 9 are not valid answers.
Only one answer to this question: some part of your code is too slow.