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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.
Did you ever get this work? I'm having the exact same problem in C. I tried returning NULL as the function's return value as the test indicated, but that didn't work. Then I turned the bad entry into a 0 (NULL), but that didn't work either.
Your solution handles some specific invalid inputs incorrectly. Read the description carefully and verify whether your solution handles all possible invali inputs.
Having said that, I agree that feedback of tests could be improved.
Bugs in your solution are not a kata issue.