mirror of
https://github.com/correl/euler.git
synced 2024-11-27 11:09:54 +00:00
32 lines
1.2 KiB
Haskell
32 lines
1.2 KiB
Haskell
{- 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)
|