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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.
Ugh... My MethodBuilder is not concurrency-safe. But I think I've made a much simpler solution than others, it's also not tied to the Thing class and can be used for any class.
Seems like your N's is to small to see benefits from Counter. Your solution is O(n^2), so for large N, your solution will be much slower than op's solution (wich is O(n))