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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.
Correct, but you could be nicer about it!
"numStr.ToCharArray().ToList()" ??? Quite the overkill there....
Select() needs an IEnumerable.
List is an IEnumerable.
char[] is an IEnumerable.
and even
string is an IEnumerable.
numStr.Select(....) would work just fine.