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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.
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Concatination in loop will create a lot of unnecessary string objects, which will affect performance. Use StringBuilder for adding Strings
Masterpiece
I am quite disappointed with the all solutions of this kata, since in my opinion the main and only difficulty was precisely in using the string builder correctly instead of string concatenation.
For example, in one of the worst cases, getPINs("55558888") will produce up to 97 thousand extra String objects because of string concatenation."