Implement a stack using single queue

We are given queue data structure, the task is to implement stack using only given queue data structure.

We have discussed a solution that uses two queues. In this article, a new solution is discussed that uses only one queue. This solution assumes that we can find size of queue at any point. The idea is to keep newly inserted element always at rear of queue, keeping order of previous elements same. Below are complete steps.

// x is the element to be pushed and s is stack
push(s, x) 
  1) Let size of q be s. 
  1) Enqueue x to q
  2) One by one Dequeue s items from queue and enqueue them.
  
// Removes an item from stack
pop(s)
  1) Dequeue an item from q

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Below is implementation of the idea.

C++

// C++ program to implement a stack using 
// single queue 
#include<bits/stdc++.h> 
using namespace std; 
  
// User defined stack that uses a queue 
class Stack 
{ 
    queue<int>q; 
public: 
    void push(int val); 
    void pop(); 
    int top(); 
    bool empty(); 
}; 
  
// Push operation 
void Stack::push(int val) 
{ 
    //  Get previous size of queue 
    int s = q.size(); 
  
    // Push current element 
    q.push(val); 
  
    // Pop (or Dequeue) all previous 
    // elements and put them after current 
    // element 
    for (int i=0; i<s; i++) 
    { 
        // this will add front element into 
        // rear of queue 
        q.push(q.front()); 
  
        // this will delete front element 
        q.pop(); 
    } 
} 
  
// Removes the top element 
void Stack::pop() 
{ 
    if (q.empty()) 
        cout << "No elements\n"; 
    else
        q.pop(); 
} 
  
// Returns top of stack 
int  Stack::top() 
{ 
    return (q.empty())? -1 : q.front(); 
} 
  
// Returns true if Stack is empty else false 
bool Stack::empty() 
{ 
    return (q.empty()); 
} 
  
// Driver code 
int main() 
{ 
    Stack s; 
    s.push(10); 
    s.push(20); 
    cout << s.top() << endl; 
    s.pop(); 
    s.push(30); 
    s.pop(); 
    cout << s.top() << endl; 
    return 0; 
} 

Java

// Java program to implement stack using a  
// single queue 
  
import java.util.LinkedList; 
import java.util.Queue; 
  
public class stack  
{ 
    Queue<Integer> q = new LinkedList<Integer>(); 
      
    // Push operation 
    void push(int val)  
    { 
        // get previous size of queue 
        int size = q.size(); 
          
        // Add current element 
        q.add(val); 
          
        // Pop (or Dequeue) all previous 
        // elements and put them after current 
        // element 
        for (int i = 0; i < size; i++)  
        { 
            // this will add front element into 
            // rear of queue 
            int x = q.remove(); 
            q.add(x); 
        } 
    } 
      
    // Removes the top element 
    int pop()  
    { 
        if (q.isEmpty())  
        { 
            System.out.println("No elements"); 
            return -1; 
        } 
        int x = q.remove(); 
        return x; 
    } 
      
    // Returns top of stack 
    int top()  
    { 
        if (q.isEmpty()) 
            return -1; 
        return q.peek(); 
    } 
      
    // Returns true if Stack is empty else false 
    boolean isEmpty()  
    { 
        return q.isEmpty(); 
    } 
  
    // Driver program to test above methods 
    public static void main(String[] args)  
    { 
        stack s = new stack(); 
        s.push(10); 
        s.push(20); 
        System.out.println("Top element :" + s.top()); 
        s.pop(); 
        s.push(30); 
        s.pop(); 
        System.out.println("Top element :" + s.top()); 
    } 
} 
  
// This code is contributed by Rishabh Mahrsee 

C#

// C# program to implement stack using a  
// single queue 
using System; 
using System.Collections.Generic; 
  
public class stack  
{ 
    Queue<int> q = new Queue<int>(); 
      
    // Push operation 
    void push(int val)  
    { 
        // get previous size of queue 
        int size = q.Count; 
          
        // Add current element 
        q.Enqueue(val); 
          
        // Pop (or Dequeue) all previous 
        // elements and put them after current 
        // element 
        for (int i = 0; i < size; i++)  
        { 
            // this will add front element into 
            // rear of queue 
            int x = q.Dequeue(); 
            q.Enqueue(x); 
        } 
    } 
      
    // Removes the top element 
    int pop()  
    { 
        if (q.Count == 0)  
        { 
            Console.WriteLine("No elements"); 
            return -1; 
        } 
        int x = q.Dequeue(); 
        return x; 
    } 
      
    // Returns top of stack 
    int top()  
    { 
        if (q.Count == 0) 
            return -1; 
        return q.Peek(); 
    } 
      
    // Returns true if Stack is empty else false 
    bool isEmpty()  
    { 
        if(q.Count == 0) 
            return true; 
        return false; 
    } 
  
    // Driver program to test above methods 
    public static void Main(String[] args)  
    { 
        stack s = new stack(); 
        s.push(10); 
        s.push(20); 
        Console.WriteLine("Top element :" + s.top()); 
        s.pop(); 
        s.push(30); 
        s.pop(); 
        Console.WriteLine("Top element :" + s.top()); 
    } 
} 
  
// This code has been contributed by Rajput-Ji  

Output :

20
10

This article is contributed by Manu Agrawal. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

My Personal Notes

Aug	SEP	Oct
	21
2018	2019	2020

Suggest a Topic

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

C#

Recommended Posts: