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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.
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No.
The problem is probably that your algorithm is to slow for the numbers used in the submit tests as they are much larger than the numbers used in the example tests.
Edit:
If your solution returns
17179869183 when n = 10000000000in a short amount of time you should probably be able to pass(Note: this is just an example of a case where iteration won't work, there are however even large tests)
Does your solution work with the example tests?
Maybe there is another corner case with the null or empty?
Did you think of corner cases?