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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.
That's because you hit stack overflow error (obviously because you tried to implement the factorial recursively), and in CW the C# test framework incorrectly reports all stack overflows as NPE.
A known CW issue, and not kata issue per se.
That happens when you're trying to perform an operation outside the domain of the operation, e.g
sqrt(-1)orlog(-1). It's clearly your code's problem ;-)This comment is hidden because it contains spoiler information about the solution