//array-based stack implementation
template <typename E> class AStack: public Stack<E>
{
private:
    int maxSize;
    int top;
    E *listArray;

public:
    AStack(int size =defaultSize)
    {
	maxSize = size;
	top =0;
	listArray = new E[size];
    }

    ~AStack()
    {
	delete [] listArray;
    }

    void clear()
    {
	top = 0;
    }

    void push(const E& it)
    {
	Assert(top != maxSize, "stack is full");
	listArray[top++] = it;
    }

    E pop()
    {
	Assert(top != 0, "stack is empty");
	return listArray[--top];
    }

    const E& topValue() const
    {
	Assert(top != 0, "stack is empty");
	return listArray[top-1];
    }

    int length() const
    {
	return top;
    }
};