// The linked list based implementation for the NumberStack ADT
// Your name here

public class LinkedNumberStack implements NumberStack
{
    // instance variable
    private LNode m_top;

    // check whether the stack is empty
    public boolean isEmpty()
    {
        if (m_top == null)
            return true;
        else
            return false;
    }

    // check whether the stack is full
    public boolean isFull()
    {
        return false;
    }

    // return the element at the top of the stack
    public int top()
    {
        if (isEmpty())
            throw new RuntimeException("top attempted on an empty stack");
        else
            return m_top.getInfo();
    }

    // push a value onto the stack
    public void push(int v)
    {
	LNode new_node = new LNode(v);
        if (isEmpty()) {
	    m_top = new_node;
	}
	else {
	    new_node.setLink(m_top);
	    m_top = new_node;
	}
    }

    // remove and return the value at the top of the stack
    public int pop()
    {
	if (isEmpty())
            throw new RuntimeException("pop attempted on an empty stack");
        else {
    	    int top_val = m_top.getInfo();
	    LNode prev_node = m_top.getLink();
	    m_top = prev_node;
	    return top_val;
	}
    }

    // return the number of elements on the stack
    public int size()
    {
	int current_size = 0;
	LNode current_node = m_top;
	while (current_node != null) {
	    current_node = current_node.getLink();
	    current_size += 1;
	}
	return current_size;
    }

    // return a string representation of the stack
    @Override
    public String toString()
    {
        String stackContent = "";
        LNode current = m_top;

        while (current != null)
	    {
		stackContent += current.getInfo() + " ";
		current = current.getLink();
	    }

        return stackContent;
    }
}