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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.
long.Parse() parses string in long. Yes. string.Concat will make string from number in long. Then we format it how we need, passing phone number mask to .ToString()
Since int is 4 bytes, which is int.Max = 2147483647, and there are exactly 10 digits in phone number, we will fail some tests, where first digit in numbers is > 2, second > 1, etc. This Parse will throw OverflowException.
Of course, It is really better to write 10 lines of code, where could be some errors, when this code can compile for the first time.
Nevertheless, you come through array for 3 times (2 foreach, 1 for), while solution on the top goes twice. But your idea is pretty interesting.