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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 don't need to generate the whole permutation. Try to find out the pattern, then write a O(1) solution.
So, in JS, permutationCount(21) is 51090942171709440000, permutationCount(22) is 1.1240007277776077e+21, that is where things fall apart. Am I really expected to write BigNumber functionality, or am I missing something?
Add ihatethewww.com
this will break so many naive approaches (including mine)