#include <iostream>

using namespace std;

class Node {
	friend ostream &operator<< (ostream&, const Node*);
public:
	int data;
	Node* next;
};

ostream &operator<< (ostream& output, const Node* cur)
{
	output << " [ " << cur->data << " ] -> ";

	if (cur->next == NULL)
		output << "||";

	return output;
}

class Stack {
	friend ostream &operator<< (ostream&, const Stack&);
public:
	Stack(void);
	~Stack();
	void Push(int);
	void Pop(int&, int&);
	int IsEmpty(void);
private:
	Node* head;
};

int Stack::IsEmpty(void)
{
	return (head == NULL);		
}

Stack::Stack(void)
{
	head = NULL;
}

Stack::~Stack()
{
	Node* t;

	while (head != NULL) {
		t = head;
		head = head->next;
		delete t;
	}
}

void Stack::Push(int i)
{
	Node* t;

	t = new Node;
	t->data = i;
	t->next = head;
	head = t;
}

void Stack::Pop(int &rv, int &err)
{
	Node* t;

	if (IsEmpty()) {
		err = 1;
		rv = 0;
	}
	else {
		err = 0;
		rv = head->data;
		t = head;
		head = head->next;
		delete t;
	}
}

ostream &operator<< (ostream& output, const Stack& c)
{
	Node* t;
	t = c.head;

	while ( t != NULL) {
		output << t;
		t = t->next;
	}

	output << endl;

	return output;
}

void main (void)
{
	Stack st;
	int rv, err;

	st.Push(42);
	cout << st;

	st.Push(31);
	cout << st;

	st.Push(59);
	cout << st;

	st.Pop(rv, err);
	cout << st;

	if (!err)
		cout << rv << endl;

	st.Pop(rv, err);
	cout << st;

	if (!err)
		cout << rv << endl;
	else
		cout << "ERROR" << endl;

	st.Pop(rv, err);
	cout << st;

	if (!err)
		cout << rv << endl;
	else
		cout << "ERROR" << endl;

	st.Pop(rv, err);
	cout << st;

	if (!err)
		cout << rv << endl;
	else
		cout << "ERROR" << endl;
}