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collatz :: Int -> [Int]
collatz 1 = [1]
collatz n
| mod n 2 == 0 = n : collatz (div n 2)
| mod n 2 == 1 = n : collatz (3*n + 1)
p14 :: Int
p14 = p14' 999999 0 0
where
p14' 0 maxnum _ = maxnum
p14' n maxnum maxlen =
let len = length (collatz n)
in if len > maxlen
then p14' (n-1) n len
else p14' (n-1) maxnum maxlen
main = putStrLn $ "Solution: " ++ show p14
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