Simple solution to https://projecteuler.net/problem=14

14.hs

module Main (main) where

col :: Int -> [Int] -> [Int]
col 1 acc = acc
col n acc | n `mod` 2 == 0 = col (n `div` 2) (n:acc)
          | otherwise  = col (3 * n + 1) (n:acc)

opts :: Int -> [(Int, Int)]
opts n = [(length(col x []), x) | x <- [1..n]]

main = do print (snd $ maximum $ opts 1000000)