Scala flatten List

问题:

I want to write a function that flats a List.

object Flat {
  def flatten[T](list: List[T]): List[T] = list match {
    case Nil => Nil
    case head :: Nil => List(head)
    case head :: tail => (head match {
      case l: List[T] => flatten(l)
      case i => List(i)
    }) ::: flatten(tail)
  }
}

object Main {
  def main(args: Array[String]) = {
    println(Flat.flatten(List(List(1, 1), 2, List(3, List(5, 8)))))
  }
}

I don't know why it don't work, it returns List(1, 1, 2, List(3, List(5, 8))) but it should be List(1, 1, 2, 3, 5, 8).

Can you give me a hint?

回答1:

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

回答2:

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)
  }

回答3:

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)
  } 

回答4:

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