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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.
I think that's what they meant, it would be more performant to use a StringBuilder in this case.
It's a concatenation to a string based on whether a character is less than character value for 5. I'm not sure where you're getting that I'm incrementing a string, += on a string is concatenation without saying string = string + "value";.
You should probably understand the code before you comment on it telling someone what you think they did wrong.
how is this best practice ? You're incrementing a string! A string builder should be used here