final sclass SimpleStack<A> extends RandomAccessAbstractList<A> {
  new ArrayList<A> l;
  int peak;
  
  public A get(int i) { ret l.get(i); }
  public int size() { ret l.size(); }

  public A set(int i, A a) { ret l.set(i, a); }
  public void add(int i, A a) { l.add(i, a); checkSize(); }
  public A remove(int i) { l.remove(i); checkSize(); }
  
  void checkSize {
    int n = size();
    if (n > peak) peak = n; // record new peak size (~actual capacity)
    if (n*2 < peak) { // shrink physically when below half of peak
      trimToSize(l);
      peak = n;
    }
  }
}