stack_from_queue.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; 
    } 
}; 
  
// driver code 
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; 
}