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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.
The substring contains the prefix, and you have to sort them in ascending lexicographic order, what's what you think it's missing there?
It's already there and next time, mark your post as having spoiler content.