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  1. import Data.List
  2.  
  3. gen2to gen [ ] = Just [ ]
  4. gen2to gen ( x : xs ) = let
  5.     lower = [ y | y <- xs, elem ( y, x ) gen ]
  6.     higher =  [ y | y <- xs, elem ( x, y ) gen ]
  7.     newgen = [ ( y, z ) | y <- lower, z <- higher ]
  8.     cont = [ ] /= ( intersect lower higher )
  9.     in if cont 
  10.         then Nothing
  11.         else do 
  12.             po' <- gen2to ( gen ++ newgen ) xs
  13.             let ( lowerS, higherS ) = break ( flip elem higher ) po'
  14.             return $ lowerS ++ [ x ] ++ higherS
  15.  
  16. main = do
  17.     print $ gen2to sample1 "ABC"
  18.     print $ gen2to  sample2 "ABCD"
  19.     print $ gen2to  sample3 "ABCDE"
  20.  
  21. sample1 = [ ]
  22.     ++ [ ( x , 'A' ) | x <- [ 'B', 'C' ] ]
  23.     ++ [ ( x , 'B' ) | x <- [ 'C' ] ]
  24.     ++ [ ( x , 'C' ) | x <- [ ] ]
  25.  
  26. sample2 = [ ]
  27.     ++ [ ( x , 'A' ) | x <- "C" ]
  28.     ++ [ ( x , 'B' ) | x <- "D" ]
  29.     ++ [ ( x , 'C' ) | x <- "BD" ]
  30.     ++ [ ( x , 'D' ) | x <- "" ]
  31.  
  32. sample3 = [ ]
  33.     ++ [ ( x , 'A' ) | x <- "E" ]
  34.     ++ [ ( x , 'B' ) | x <- "D" ]
  35.     ++ [ ( x , 'C' ) | x <- "B" ]
  36.     ++ [ ( x , 'D' ) | x <- "" ]
  37.     ++ [ ( x , 'E' ) | x <- "B" ]
  38.  
Success #stdin #stdout 0.01s 5472KB
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Standard input is empty
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Just "CBA"
Just "DBCA"
Just "DBECA"
https://ideone.com/3eleC6
language:
Haskell (ghc 8.4.4)
created:
3 years ago
visibility:
public

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