24 lines
847 B
Haskell
24 lines
847 B
Haskell
-- The prime factors of 13195 are 5, 7, 13 and 29. --
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-- What is the largest prime factor of the number 600851475143?
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isPrime :: (Integral n) => n -> Bool
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isPrime 1 = False
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isPrime 2 = True
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isPrime 3 = True
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isPrime n =
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n `mod` 2 /= 0 && n `mod` 3 /= 0 && null [ x | x <- candidates, n `mod` x == 0 || n `mod` (x+2) == 0 ]
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where candidates = [5,11..limit]
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limit = floor(sqrt(fromIntegral n)) + 1
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maxPrimeFactor :: (Integral n) => n -> n
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maxPrimeFactor n
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| isPrime n = n
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| n `mod` 2 == 0 = maxPrimeFactor $ fromIntegral n `div` 2
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| otherwise = maxPrimeFactor $ fromIntegral n `div` head [i | i <- [3,5..], n `mod` i == 0 && isPrime i]
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main :: IO ()
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main = do
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let result = maxPrimeFactor 600851475143
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putStrLn $ "Project Euler, Problem 3\n"
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++ "Answer: " ++ (show result)
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