// Please replace the deleted * and -> symbols
// debugging function added to demonstrate how links work.
#include <iostream>
using namespace std;

#include "stack.h"

class Element{
   friend class Stack;
   Element next;
   int data;
   Element( Element *n, int d): next(n), data(d){}
 public:
   void debug(){ cerr << (unsigned int)this << ":Element[next="<<(unsigned int)next<<", data="
		<< data << "]\n"; }
};

void Stack::debug()//nonstandard
{
	cerr << (unsigned int)this <<":Stack[first="<<(unsigned int)first<<"]"<< endl;
	for(Element * p = first; p!=NULL; p=p->next)
		p->debug();
	cerr << "---------------\n";
}

void Stack::push(int d)
{
   first = new Element(first, d);
}
void Stack::pop() throw(length_error)
{
    if( empty() )
       throw length_error("Stack::pop");
    Element p=first;
    int d = p data;
    first = first next;
    delete p;
}
int Stack::top() throw(length_error)
{
   if( empty() )
       throw length_error("Stack::pop");
   return first data;
}
bool Stack::empty()
{
	return !first;
}
Stack::~Stack()
{
    while(!empty() )
    {
      Element p=first;
      first = first next;
      delete p;
    }
}