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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.
Learn to match! The code works in O(n), this is a naive solution. If the input data value is greater than 10 ^10, the code will work very slowly (or even not at all). Using the sum of the arithmetic progression gives a constant time complexity. I don't understand why so many people marked such a decision as "the best".