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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.
Some of these solutions, even though they pass the tests, only returns all zeros or all ones. So they aren't really solutions to the problem.
I wonder if it's slower to concatenate "_" than append "_" on a leftover because the whole string has to be traversed