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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.
xor is very clever.
I think this would be faster than sorting. Assuming sorting takes O(nlogn) time, and removal takes O(n) time, O(n) < O(nlogn), hence it would be more efficient. Since we shouldn't modify the original array we can make a copy.