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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.
nice version, but in some cases better to use eratosthenes sieve for precomputing prime numbers
yeah, you can use additional array or just sort. Also you can try not sqrt(N), but 2 * (cubed root of N). As i remember it also will work.