import org.scalatest.FlatSpec

class FlattenTest extends FlatSpec {
  "Empty List" should "should flatten to empty list" in {
    val empty = List()
    assert(FlattenUtility.Flatten(empty) == List())
  }

  "Flat list" should "return flat list" in {
    val flat = List(1, 2, 3, 4)
    assert(FlattenUtility.Flatten(flat) == flat)
  }

  "Deep list" should "should return flat list" in {
    val deep = List(List(List(1)))
    assert(FlattenUtility.Flatten(deep) == List(1))
  }

  "Deep and flat list" should "should return flat list" in {
    val both = List(List(1), 2, 3, List(4, List(5)))
    assert(FlattenUtility.Flatten(both) == List(1, 2, 3, 4, 5))
  }
}

object FlattenUtility {
  def Flatten(toFlatten : Any) : List[Any] = {
    return toFlatten match {
      case first :: second :: rest => Flatten(first) ++ Flatten(second :: rest)
      case first :: _ => Flatten(first)
      case List() => List()
      case t : Any => List(t)
    }
  }
}