Scala - convert List of Lists into a single List: List[List[A]] to List[A]

Question

What\'s the best way to convert a List of Lists in scala (2.9)?

I have a list:

List[List[A]]

which I want to convert into

Answer 1

You don't need recursion but you can use it if you want:

def flatten[A](list: List[List[A]]):List[A] = 
  if (list.length==0) List[A]() 
  else list.head ++ flatten(list.tail)

This works like flatten method build into List. Example:

scala> flatten(List(List(1,2), List(3,4)))
res0: List[Int] = List(1, 2, 3, 4)