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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.
Sometimes creating a list for the purpose of calling sum is marginally faster (especially if the list gets longer)
Hence I upvoted this answer along with a very similar one that uses map
This converts the number toString K times where K is the number of digits in n