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Diffstat (limited to 'haskell/024-lexicographic_premutations.hs')
| -rw-r--r-- | haskell/024-lexicographic_premutations.hs | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/haskell/024-lexicographic_premutations.hs b/haskell/024-lexicographic_premutations.hs new file mode 100644 index 0000000..5db0aaa --- /dev/null +++ b/haskell/024-lexicographic_premutations.hs @@ -0,0 +1,21 @@ +-- Lexicographic permutations +-- +-- Problem 24 +-- A permutation is an ordered arrangement of objects. For example, 3124 is one possible +-- permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically +-- or alphabetically, we call it lexicographic order. The lexicographic permutations of +-- 0, 1 and 2 are: +-- +-- 012 021 102 120 201 210 +-- +-- What is the millionth lexicographic permutation of the digits +-- 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9? + + +import Data.Char(chr,ord) + +main = print (foldr (\x acc -> chr (ord '0' + x) : acc) "" ((permutations' [0..9]) !! 999999)) + +permutations' :: [Int] -> [[Int]] +permutations' [x] = [[x]] +permutations' xs = concat [[x : p | p <- permutations' (filter (x/=) xs)] | x <- xs] |