Project Euler 解答

Project Euler Problem 004

Project Euler Problem 004

import Data.List
import Control.Applicative
ispalin :: Eq a => [a] -> Bool
ispalin str = str == reverse str
from :: Integer
from=999
list :: [(Integer, Integer)]
list = [(x,y)|x<-[1..from],y<-[1..from]]
palinlist :: [(Integer, Integer)]
palinlist = filter (\(x,y) -> ispalin $ show (x*y)) list 
sortedlist :: [(Integer, Integer)]
sortedlist = sortBy (\(l,r) (ll,rr) -> compare (l*r) (ll*rr)) palinlist
answer :: Integer
answer = (liftA2 (*) fst snd) $ last sortedlist
-- 906609
main :: IO ()
main = print answer

-- 別解
d :: [Integer]
d = [100..999]
m :: (Integer, Integer, Integer)
m = maximum [(t,a,b) | a<-d,b<-d,let t=a*b,reverse(show t)==show t] --(906609,993,913)

since 2013