class ArrayBasedStack {
  public Object[] stack;
  public int size;

  /**
   * The constructor of the ArrayBasedStack.
   *
   * @param capacity
   *     Maximum number of items that can be stored in the stack.
   */
   public ArrayBasedStack(int capacity) {
    stack = new Object[capacity];
    size = 0;
  }

  /**
   * Push an item to the stack.
   *
   * @param o
   *     Object that is pushed on the stack.
   * @return the object, iff the object has been pushed to stack successfully.
   * Returns null, if the object cannot be pushed to the stack.
   */
  public Object push(Object o) {
    if(size == stack.length) {
    	return null;
    }
    stack[size++] = o;
    return o;

  }

  /**
   * Pops the first item from the stack.
   *
   * @return the object, iff the object has been popped from stack successfully. Null, if the stack is empty.
   */
  public Object pop() {
	  if(size == 0)
		  return null;
	  Object o = stack[size-1];
	  size--;
	  return o;
  }

  /**
   * Check the item on top of the stack, without removing it.
   *
   * @return the top object, iff the stack has an item. Null, if the stack is empty.
   */
  public Object peek() {
    if(size == 0) {
    	return null;
    }
    return stack[size-1];
  }
}