ITI1121 exam question from 2013/2014, implemented

AStack.java

class AStack implements Stack {
  public static final int MAX_ITEMS = 1000;

  public static final int ITERATIONS = 9;

  private int items[] = new int[MAX_ITEMS];
  private int i = 0;

  public void push(int item) {
    if (i < MAX_ITEMS) {
      items[i++] = item;
    }
  }

  public int pop() {
    if (i <= 0)
      return Integer.MIN_VALUE; // this could be any garbage number, or we can cause an error and crash the program
    return items[--i];
  }

  public boolean isEmpty() {
    return (i == 0);
  }

  public static boolean isSkipped(Stack s1, Stack s2) {
    boolean return_value = false;
    boolean complete = false;
    int z1_element, z2_element;
    Stack z1, z2;

    if (s1.isEmpty() && s2.isEmpty())
      return true;

    z1 = new AStack();
    z2 = new AStack();

    // reverse s1 and s2 into z1 and z2 respectively
    while (!s1.isEmpty()) {
      z1.push(s1.pop());
    }
    while (!s2.isEmpty()) {
      z2.push(s2.pop());
    }

    // the first element should be the same in both z1 and z2
    z1_element = z1.pop();
    z2_element = z2.pop();
    s1.push(z1_element);
    s2.push(z2_element);

    if (z1_element != z2_element) {
      // here the first element in z1 and z2 is not the same
      // therefore, isSkipped → false
      complete = true;
    }

    while (!complete) {

      if (z1.isEmpty()) {
        if (z2.isEmpty()) {
          return_value = true;
        } // else: return_value = false;
        complete = true;
        break;
      }

      // every second element in z1 (after the first) should be the same as the next element in z2
      s1.push(z1.pop());

      if (z2.isEmpty()) {
        if (z1.isEmpty()) {
          return_value = true;
        } // else: return_value = false; 
        complete = true;
        break;
      }

      z1_element = z1.pop();
      z2_element = z2.pop();
      s1.push(z1_element);
      s2.push(z2_element);

      if (z1_element != z2_element) {
        complete = true;
        break;
      }

    }


    while (!z1.isEmpty()) {
      s1.push(z1.pop());
    }
    while (!z2.isEmpty()) {
      s2.push(z2.pop());
    }

    return return_value;
  }

  public static void main(String[] args) {
    Stack s1, s2;
    s1 = new AStack();
    s2 = new AStack();

    fillStacks(s1,s2);
    for (int i = 1; i < ITERATIONS; ++i) {
      System.out.printf("%d\t s1: %d \t s2: %d\n", i, s1.pop(), s2.pop());
    }

    fillStacks(s1,s2);
    System.out.println("\nisSkipped: " + AStack.isSkipped(s1, s2) + "\n");

    for (int i = 1; i < ITERATIONS; ++i) {
      System.out.printf("%d\t s1: %d \t s2: %d\n", i, s1.pop(), s2.pop());
    }
  }

  public static void fillStacks(Stack s1, Stack s2){

    for (int i = 1; i < ITERATIONS; ++i) {
      int data = i + 10000;
      s1.push(data);
        if (i % 2 != 0)
            s2.push(data);
    }

  }
}

Stack.java

package guy;
public interface Stack {
  void push(int item);
  int pop();
  boolean isEmpty();
}