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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.
You are right. Tried with profiler on long ints (10k-5m digits) - the most time consuming operation inevitably is building the result and your way (from string) is way faster.
Just realized that int operations is not O(1) anymore when python unlimited ints is involved.