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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.
Oh, thank you for pointing that out!
The kata description says: "The input string can be assumed to contain only alphabets (both uppercase and lowercase) and numeric digits."
Why don't you check whether a character is digit or alphabetic?
Maybe, tests don't consider this case or I just don't understand something obvious...
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