package collections;

interface Stack {
  public void push(int elt);
  public int pop();
  public boolean isEmpty();
}
public class ArrayStack implements Stack{
  private final int MAX_ELEMENTS = 10;
  private int[] stack;
  private int index;
  public ArrayStack() {
    stack = new int[MAX_ELEMENTS];
    index = -1;
  }
  public void push(int elt) {
    if (index != stack.length - 1) {
      index++;                                        //1
      stack[index] = elt;                             //2
    } else {
      throw new IllegalStateException("stack overflow");
    }
  }
  public int pop() {
    if (index != -1) {
      return stack[index--];
    } else {
      throw new IllegalStateException("stack underflow");
    }
  }
  public boolean isEmpty() { return index == -1; }
}