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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.
Because the core tests contain random tests, to make sure your code always works. If you do not pass, there must be some inputs which cause your solution to fail. :)
Maybe this will help:
https://docs.codewars.com/training/troubleshooting#test-attempt