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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.
What if there are more than 4 numbers?
This seems like it would have some performance issues though; you are creatign a new String everytime you do res+= i.