Project Euler 解答

Project Euler Problem 045

Project Euler Problem 045

import Data.List
import Data.Maybe
-- 逆数を利用した判定
istrian :: Integer -> Bool
istrian n = r - (fromIntegral $ floor r) < 1e-5 where r = (sqrt((fromIntegral n)*8.0+1.0)-1.0)/2.0
ispenta :: Integer -> Bool
ispenta n = r - (fromIntegral $ floor r) < 1e-5 where r = (sqrt((fromIntegral n)*24+1.0)+1.0)/6.0
ishexag :: Integer -> Bool
ishexag n = r - (fromIntegral $ floor r) < 1e-5 where r = (sqrt((fromIntegral n)*8.0+1.0)+1.0)/4.0
-- 六角数のリスト。六角数は三角数でもある
hexas :: [Integer]
hexas = map (\n-> n*(2*n-1)) [144..]
ans1 :: Integer
ans1 = fromJust $ find ispenta hexas
-- 1533776805
main :: IO ()
main = print ans1

since 2013