object Concatenate { def concat[T](l: List[List[T]]): List[T] = FoldLeft.foldLeft(l, Nil: List[T])(AppendByFoldLeft.append) def main(args: Array[String]): Unit = { println(concat(List(List(1, 2, 3), List(4, 5, 6), List(7, 8, 9)))) } }
List(1, 2, 3, 4, 5, 6, 7, 8, 9)