def zipWith[A, B](lists: List[List[A]])(f: List[A] => B): List[B] = {
    @tailrec
    def loop(acc: List[B], input: List[List[A]]): List[B] = {
      val init = (Nil: List[List[A]], Nil: List[A])
      val (left, zipped) = input.foldLeft(init)((z, a) => {
        val (tails, heads) = z
        (a.tail :: tails, a.head :: heads)
      })
      if (left.foldLeft(false)(_ || _.isEmpty))
        f(zipped.reverse) :: acc
      else
        loop(f(zipped.reverse) :: acc, left.reverse)
    }

    loop(Nil, lists).reverse
  }

  test("zipWith"){
    assert(zipWith(List(List(1,2,3), List(4,5,6), List(7,8,9)))(_.mkString("::")) === List("1::4::7", "2::5::8", "3::6::9"))
  }