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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.
Nested loop alters the outer loop counter so it 'fasts forward', it therefore does not cause combinatorial wasted effort.
Your code has ~4 extra conditionals so is unlikely to perform any better.
that's a clever way to do it :)
At the return, you could use sb.substring(0, sb.length() - 1); and it returns a string, so you dont have to use toString()