Stack using queues.cpp

/* Program to implement a stack using 
two queue */
#include <bits/stdc++.h> 

using namespace std; 

class Stack { 
	// Two inbuilt queues 
	queue<int> q1, q2; 

	// To maintain current number of 
	// elements 
	int curr_size; 

public: 
	Stack() 
	{ 
		curr_size = 0; 
	} 

	void push(int x) 
	{ 
		curr_size++; 

		// Push x first in empty q2 
		q2.push(x); 

		// Push all the remaining 
		// elements in q1 to q2. 
		while (!q1.empty()) { 
			q2.push(q1.front()); 
			q1.pop(); 
		} 

		// swap the names of two queues 
		queue<int> q = q1; 
		q1 = q2; 
		q2 = q; 
	} 

	void pop() 
	{ 

		// if no elements are there in q1 
		if (q1.empty()) 
			return; 
		q1.pop(); 
		curr_size--; 
	} 

	int top() 
	{ 
		if (q1.empty()) 
			return -1; 
		return q1.front(); 
	} 

	int size() 
	{ 
		return curr_size; 
	} 
}; 

int main() 
{ 
	Stack s; 
	s.push(1); 
	s.push(2); 
	s.push(3); 

	cout << "current size: " << s.size() 
		<< endl; 
	cout << s.top() << endl; 
	s.pop(); 
	cout << s.top() << endl; 
	s.pop(); 
	cout << s.top() << endl; 

	cout << "current size: " << s.size() 
		<< endl; 
	return 0; 
}