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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.
This is irrelevent, as the kata only concerns itself with arrays that are expected to always have exactly one unique element.
try [1,1,1,2,2] :( will not work correctly to find a unique number because the array does not have a single unique element; it contains two numbers that each appear multiple times.