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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.
Of course you don't NEED to iterate through. It's a really quick solution to a really quick problem, that's why it's good. The Intstream solution takes 1.3s if the input is 1 billion, but since the tests were way smaller, that ain't a problem. If coding a real project, then of course everyone should think twice if their solution is really the most efficient or not. Here it's not necessary.