euler_014.scala

def collatz(start: Long) = {
  def collatz_i(chain: List[Long]): List[Long] = {
    if (chain.head == 1) chain
    else if (chain.head % 2 == 0) collatz_i((chain.head / 2) :: chain)
    else collatz_i((chain.head * 3 + 1) :: chain)
  }
  collatz_i(start :: Nil)
}
(2 until 1000000).foldLeft((1,collatz(1).size)){(acc,next) =>
  val chainSize = collatz(next).size
  if (chainSize > acc._2) (next, chainSize) else acc
}._1