Implementing Stack using two Queues. Push - O(n), Pop - O(1)

Stack.scala

import scala.collection.immutable.Queue

class Stack[+A](val in: Queue[A] = Queue.empty[A], val out: Queue[A] = Queue.empty[A]) {

  def push[B >: A](v: B): Stack[B] = {
    var i = in.enqueue(v)
    var o = out
    while (!o.isEmpty) {
      val (vv, oo) = o.dequeue
      o = oo
      i = i.enqueue(vv)
    }
    new Stack(o, i)
  }

  def pop: (A, Stack[A]) = { val (v, o) = out.dequeue; (v, new Stack(in, o)) }
}