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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.
So that's why I'm seeing StringBuilder a lot!
this solution is helpful for newbie
a solution with very poor performance as each addition will create a new string,
because of that problem StringBuilder was invented
I am not saying to use StringBuilder, but your solution is certainly not performant