Haskell 004

haskell/e004.hs

@@ -0,0 +1,32 @@
+{- Find the largest palindrome made from the product of two 3-digit numbers.
+
+A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91  99.
+Find the largest palindrome made from the product of two 3-digit numbers.
+-}
+
+import Data.List (nub, sort)
+import Text.Printf
+
+is_palindrome :: (Integral a) => a -> Bool
+is_palindrome n = do
+    let s = (show n)
+    s == (reverse s)
+
+largest_palindrome :: (Integral a) => a -> a
+largest_palindrome digits = do
+    let min = 10 ^ (digits - 1)
+    let max = 10 ^ digits - 1
+    largest_palindrome_ min max max max 0
+
+largest_palindrome_ :: (Integral a) => a -> a -> a -> a -> a -> a
+largest_palindrome_ min max a b p
+    | a < min = p
+    | a * b < p || b < min = largest_palindrome_ min max (a - 1) max p
+    | otherwise =
+        if is_palindrome (a * b)
+            then largest_palindrome_ min max a (b - 1) (a * b)
+            else largest_palindrome_ min max a (b - 1) p
+
+main = do
+    printf "Largest palindrome product of 2-digit numbers: %d\n" (largest_palindrome 2 :: Integer)
+    printf "Largest palindrome product of 3-digit numbers: %d\n" (largest_palindrome 3 :: Integer)