flatten.scala

// using flatMap
def flatten(xs: List[Any]): List[Any] = xs.flatMap {
    case ls: List[_] => flatten(ls)
    case x => List(x)
}

// without flatMap
def flatten(xs: List[Any]): List[Any] = xs.foldLeft(List[Any]()) {
    case (acc, ls: List[_]) => acc ::: flatten(ls)
    case (acc, x) => acc ::: List(x)
}