Created
September 28, 2019 03:15
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| derangements :: Integer -> Integer | |
| derangements n = id | |
| . sum | |
| . zipWith (*) signs | |
| . scanl (*) 1 | |
| $ [n,n-1..1] | |
| where | |
| signs = drop (n .&. 1) (cycle [1,-1]) |
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