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33 lines
1.2 KiB
Haskell
33 lines
1.2 KiB
Haskell
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{- Find the largest palindrome made from the product of two 3-digit numbers.
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A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99.
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Find the largest palindrome made from the product of two 3-digit numbers.
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-}
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import Data.List (nub, sort)
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import Text.Printf
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is_palindrome :: (Integral a) => a -> Bool
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is_palindrome n = do
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let s = (show n)
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s == (reverse s)
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largest_palindrome :: (Integral a) => a -> a
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largest_palindrome digits = do
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let min = 10 ^ (digits - 1)
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let max = 10 ^ digits - 1
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largest_palindrome_ min max max max 0
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largest_palindrome_ :: (Integral a) => a -> a -> a -> a -> a -> a
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largest_palindrome_ min max a b p
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| a < min = p
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| a * b < p || b < min = largest_palindrome_ min max (a - 1) max p
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| otherwise =
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if is_palindrome (a * b)
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then largest_palindrome_ min max a (b - 1) (a * b)
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else largest_palindrome_ min max a (b - 1) p
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main = do
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printf "Largest palindrome product of 2-digit numbers: %d\n" (largest_palindrome 2 :: Integer)
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printf "Largest palindrome product of 3-digit numbers: %d\n" (largest_palindrome 3 :: Integer)
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