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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.
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this approach is actually especially bad in terms of performances. you cannot "omptimize" that, you have to completely change the algorithm.
Try to imagine how you would solve it by hand, but on very long strings. Then, try to implement that.