Implement Stack using Queues

The problem is opposite of this post. We are given a Queue data structure that supports standard operations like enqueue() and dequeue(). We need to implement a Stack data structure using only instances of Queue and queue operations allowed on the instances.

Stack and Queue with insert and delete operations

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

A stack can be implemented using two queues. Let stack to be implemented be ‘s’ and queues used to implement be ‘q1’ and ‘q2’. Stack ‘s’ can be implemented in two ways:

Method 1 (By making push operation costly)
This method makes sure that newly entered element is always at the front of ‘q1’, so that pop operation just dequeues from ‘q1’. ‘q2’ is used to put every new element at front of ‘q1’.

push(s, x) // x is the element to be pushed and s is stack
  1) Enqueue x to q2
  2) One by one dequeue everything from q1 and enqueue to q2.
  3) Swap the names of q1 and q2 
// Swapping of names is done to avoid one more movement of all elements 
// from q2 to q1. 

pop(s)
  1) Dequeue an item from q1 and return it.

C++

/* 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; 
} 
// This code is contributed by Chhavi  

Java

/* Java Program to implement a stack using  
two queue */
import java.util.*; 
  
class GfG { 
  
static class Stack  
{  
    // Two inbuilt queues  
    static Queue<Integer> q1 = new LinkedList<Integer>(); 
    static Queue<Integer> q2 = new LinkedList<Integer>();  
      
    // To maintain current number of  
    // elements  
    static int curr_size;  
  
    Stack()  
    {  
        curr_size = 0;  
    }  
  
    static void push(int x)  
    {  
        curr_size++;  
  
        // Push x first in empty q2  
        q2.add(x);  
  
        // Push all the remaining  
        // elements in q1 to q2.  
        while (!q1.isEmpty())  
        {  
            q2.add(q1.peek());  
            q1.remove();  
        }  
  
        // swap the names of two queues  
        Queue<Integer> q = q1;  
        q1 = q2;  
        q2 = q;  
    }  
  
    static void pop(){  
  
        // if no elements are there in q1  
        if (q1.isEmpty())  
            return ;  
        q1.remove();  
        curr_size--;  
    }  
  
    static int top()  
    {  
        if (q1.isEmpty())  
            return -1;  
        return q1.peek();  
    }  
  
    static int size()  
    {  
        return curr_size;  
    }  
};  
  
// driver code  
public static void main(String[] args)  
{  
    Stack s = new Stack();  
    s.push(1);  
    s.push(2);  
    s.push(3);  
  
    System.out.println("current size: " + s.size());  
    System.out.println(s.top());  
    s.pop();  
    System.out.println(s.top());  
    s.pop();  
    System.out.println(s.top());  
  
    System.out.println("current size: " + s.size());  
} 
}  
// This code is contributed by Prerna  

Output :

current size: 3
3
2
1
current size: 1

Method 2 (By making pop operation costly)
In push operation, the new element is always enqueued to q1. In pop() operation, if q2 is empty then all the elements except the last, are moved to q2. Finally the last element is dequeued from q1 and returned.

push(s,  x)
  1) Enqueue x to q1 (assuming size of q1 is unlimited).

pop(s)  
  1) One by one dequeue everything except the last element from q1 and enqueue to q2.
  2) Dequeue the last item of q1, the dequeued item is result, store it.
  3) Swap the names of q1 and q2
  4) Return the item stored in step 2.
// Swapping of names is done to avoid one more movement of all elements 
// from q2 to q1.

/* Program to implement a stack  
using two queue */
#include<bits/stdc++.h> 
using namespace std; 
  
class Stack 
{ 
    queue<int> q1, q2; 
    int curr_size; 
  
    public: 
    Stack() 
    { 
        curr_size = 0; 
    } 
  
    void pop() 
    { 
        if (q1.empty()) 
            return; 
  
        // Leave one element in q1 and  
        // push others in q2. 
        while (q1.size() != 1) 
        { 
            q2.push(q1.front()); 
            q1.pop(); 
        } 
  
        // Pop the only left element  
        // from q1 
        q1.pop(); 
        curr_size--; 
  
        // swap the names of two queues 
        queue<int> q = q1; 
        q1 = q2; 
        q2 = q; 
    } 
  
    void push(int x) 
    { 
        q1.push(x); 
        curr_size++; 
    } 
  
    int top() 
    { 
        if (q1.empty()) 
            return -1; 
  
        while( q1.size() != 1 ) 
        { 
           q2.push(q1.front()); 
           q1.pop(); 
        }  
          
        // last pushed element 
        int temp = q1.front(); 
          
        // to empty the auxiliary queue after 
        // last operation 
        q1.pop(); 
       
        // push last element to q2 
        q2.push(temp); 
  
        // swap the two queues names 
        queue<int> q = q1; 
        q1 = q2; 
        q2 = q; 
        return temp; 
    } 
  
    int size() 
    { 
        return curr_size; 
    } 
}; 
  
// driver code 
int main() 
{ 
    Stack s; 
    s.push(1); 
    s.push(2); 
    s.push(3); 
    s.push(4); 
  
    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; 
} 
// This code is contributed by Chhavi 

Output :

current size: 4
4
3
2
current size: 2

References:
Implement Stack using Two Queues

This article is compiled by Sumit Jain and reviewed by GeeksforGeeks team. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

My Personal Notes

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C++

Java

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