 Solution

class ArrayDoubleStack(val MAX: Int) extends DoubleStack {
  require(MAX >= 0)
  private val data = new Array[Int](MAX)
  private var left = 0
  private var right = MAX

  def isEmptya: Boolean = left == 0
  def isEmptyb: Boolean = right == MAX

  def pusha(x: Int): Unit = {
    require(left < right, "double stack is full")
    data(left) = x
    left += 1
  }
  def pushb(x: Int): Unit = {
    require(left < right, "double stack is full")
    right -= 1
    data(right) = x
  }
  def popa: Int = {
    require(left > 0, "stack a is empty")
    left -= 1
    data(left)
  }
  def popb: Int = {
    require(right < MAX, "stack b is empty")
    val answer = data(right)
    right += 1
    answer
  }
}