Project Euler 解答

Project Euler Problem 014

Project Euler Problem 014

import Data.Array
import Data.Array.ST
import Control.Monad.ST
 
n :: Integer
n = 10^(6::Integer)-1
sol1 :: Array Integer Integer
sol1 = a where
  a = runSTArray $
      do ar <- (newListArray (1,n) (1:[col x | x<-[2..n]])) :: ST s (STArray s Integer Integer)
         return ar
  col x | even x = if r<=n then 1+a!r else 1 + col r where r = x`quot`2
  col x = if r<=n then 1+a!r else 1 + col r where r = 3*x+1
-- (525,837799)                                                  
ans1 :: (Integer, Integer)
ans1 = maximum $ zip (elems sol1) [1..]

-- 837799
main :: IO ()
main = print $ snd ans1

since 2013