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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.
That doesn't make the complexity exponential, that would just make it O(n + n), which is still just O(n). O(n^2) would be if there was a loop that went through n and a nested loop that always went through n, or something similar. Maybe you mean that .join would be faster than +=.
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