Scala flatten List

By delete line 4

case head :: Nil => List(head)

You will get right answer.

Think about the test case

List(List(List(1)))

With line 4 last element in list will not be processed

  def flatten(ls: List[Any]): List[Any] = ls flatMap {
    case ms: List[_] => flatten(ms)
    case e => List(e)
  }

You don't need to nest your match statements. Instead do the matching in place like so:

  def flatten(xs: List[Any]): List[Any] = xs match {
    case Nil => Nil
    case (head: List[_]) :: tail => flatten(head) ++ flatten(tail)
    case head :: tail => head :: flatten(tail)
  }

My, equivalent to SDJMcHattie's, solution.

  def flatten(xs: List[Any]): List[Any] = xs match {
    case List() => List()
    case (y :: ys) :: yss => flatten(y :: ys) ::: flatten(yss)
    case y :: ys => y :: flatten(ys)
  }