 Solution

private def addFirst(x: Int): Unit = {
  val q = new Node(x)
  q.next = first
  if (first == null) last = q else first.prev = q
  first = q
  size += 1
}

private def addLast(x: Int): Unit = {
  val q = new Node(x)
  q.prev = last
  if (last == null) first = q else last.next = q
  last = q
  size += 1
}

def append(x: Int): Unit = {
  if (backwards) addFirst(x) else addLast(x)
}

def prepend(x: Int): Unit = {
  if (backwards) addLast(x) else addFirst(x)
}

def get(i: Int): Int = {
  require(0 <= i && i < size)
  val j = if (backwards) size - 1 - i else i
  var q: Node = null
  if (j <= size - 1 - j) {
    q = first
    var k = 0
    while (k < j) { q = q.next; k += 1 }
  } else {
    q = last
    var k = size - 1
    while (k > j) { q = q.prev; k -= 1 }
  }
  val x = q.value
  if (q.prev == null) first = q.next else q.prev.next = q.next
  if (q.next == null) last = q.prev else q.next.prev = q.prev
  q.prev = null
  q.next = null
  size -= 1
  x
}

def rev: Unit = { backwards = !backwards }