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  1. import System.Environment (getArgs)
  2.  
  3. data Board = Board {
  4.   field :: [Int],
  5.   size :: Int
  6. }
  7.  
  8. -- | Checks whether the board has a conflict
  9. conflict :: Board -> Bool
  10. conflict b = any (==x) xs || step (-1) x xs || step 1 x xs
  11.   where (x:xs) = field b
  12.         step _ _ [] = False
  13.         step s p (x:xs)
  14.           | (s+p) == x = True
  15.           | otherwise  = step (s+p) p xs
  16.  
  17. -- | Returns all boards one can get by placing a queen in the next column
  18. place :: Board -> [Board]
  19. place b = place' b 1
  20.   where place' b s
  21.           | s > size b        = []
  22.           | conflict newboard = rest
  23.           | otherwise         = newboard:rest
  24.           where newboard = Board newfield (size b)
  25.                 newfield = s:(field b)
  26.                 rest     = place' b (s+1)
  27.  
  28. -- | Returns all boards one can get for the n-th queen problem
  29. placeAll :: Int -> [Board]
  30. placeAll n = foldl (>>=) [Board [] n] (replicate n place)
  31.  
  32. -- | Creates a String from a board
  33. prettyPrint :: Board -> String
  34. prettyPrint b = unlines.map prettyLine.field$b
  35.   where prettyLine n = replicate (n - 1) '.' ++ "@" ++ replicate ((size b) - n) '.'
  36.  
  37. -- | Shows the first two solutions obtained by placeAll
  38. main = do
  39.   mapM_ putStrLn . map prettyPrint . take 2 . placeAll $ 44
  40.  
Success #stdin #stdout 0s 6264KB
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https://ideone.com/hgky6N
language:
Haskell (ghc 8.4.4)
created:
11 years ago
visibility:
public

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