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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.
Bugs in your solution are not a kata issue.
How do you think, what is a correct answer for input
"AB"? What does your solution return?