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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 believe this solution is wrong (and if it is, then the test cases are too).
Taking as an example the number: "12345678987654321", your solution yields the following products:
There the last value is 120 when it should be 24 (4 * 3 * 2 * 1). The problem resides I think in the for-condition.