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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.
it's Theta(2^n) and using a for instead of recursion is not that evil optimization
You don't need to do it at all, the answer can be computed with simple math.
If you're on codewars to argue efficiency, you're probably in the wrong place.
Premature optimization is the root of all evil - elegance and simplicity hold infinitely more value here.
O(2^n) is not very clever, try to run this if n is 40 or bigger... One liners are nice but they should be so slow.
This solution is O(2^n) so it's very slow also for small test cases (n = 40 for example)