Find all permutations of a Number in Scala with memoization (optimal)

permutations.scala

def remove(num: Int, list: List[Int]) = list diff List(num)

var mem: Map[List[Int], List[List[Int]]] = Map()

def permutations(l:List[Int]):List[List[Int]] =
    if (l.length == 1) List(l)
    else {
      if (mem.contains(l)) mem(l)
        else {
        val r = l.flatMap(x=> permutations(remove(x, l)).map(y=> x::y))
        mem += ((l,r))
        r
    }
}