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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 had the same thing happen. It turns out that number with the dashes was not the number provided by the test. When I was breaking the long arg into individual digits, I was doing (int) n%10 instead of (int) (n%10), which was not giving the desired digits (with some being negative numbers), causing the stack trace later on.