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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.
Pure genius!
Correction:
String.joindoes use theStringBuilderlibrary, becauseStringJoinerusesStringBuilderunder the hood. This means thatString.join,StringJoiner, andStringBuilderare all O(n) optimal ways to concatenate strings, whereas the string concatenation operator+is O(n^2).What if there are more than 4 numbers?