dailycoding14.hs

import Data.List

collatz :: Int -> [Int]
collatz 1 = [1]
collatz x
    | even x = x : collatz (x `div` 2)
    | odd  x = x : collatz (3 * x + 1)

collatzLength :: Int -> (Int, Int)
collatzLength x = (x, length (collatz x))

maxCollatzLength :: [(Int, Int)] -> (Int, Int)
maxCollatzLength xs = foldl' (\acc x -> if snd acc > snd x then acc else x) (0, 0) xs

main = print $ fst $ maxCollatzLength $ map collatzLength [1..(1000000 - 1)]