Reversing a Queue

Give an algorithm for reversing a queue Q. Only the following standard operations are allowed on queue. 

1. enqueue(x): Add an item x to the rear of the queue.
2. dequeue(): Remove an item from the front of the queue.
3. empty(): Checks if a queue is empty or not.

The task is to reverse the queue.

Examples:

Input: Q = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
Output: Q = [100, 90, 80, 70, 60, 50, 40, 30, 20, 10]

Input: [1, 2, 3, 4, 5]
Output: [5, 4, 3, 2, 1]

Reversing a Queue using stack:

For reversing the queue one approach could be to store the elements of the queue in a temporary data structure in a manner such that if we re-insert the elements in the queue they would get inserted in reverse order. So now our task is to choose such a data structure that can serve the purpose. According to the approach, the data structure should have the property of ‘LIFO’ as the last element to be inserted in the data structure should actually be the first element of the reversed queue. 

Follow the below steps to implement the idea:

• Pop the elements from the queue and insert into the stack now topmost element of the stack is the last element of the queue.
• Pop the elements of the stack to insert back into the queue the last element is the first one to be inserted into the queue.

Below is the implementation of above approach:

C++

// CPP program to reverse a Queue
#include <bits/stdc++.h>
using namespace std;

// Utility function to print the queue
void Print(queue<int>& Queue)
{
    while (!Queue.empty()) {
        cout << Queue.front() << " ";
        Queue.pop();
    }
}

// Function to reverse the queue
void reverseQueue(queue<int>& Queue)
{
    stack<int> Stack;
    while (!Queue.empty()) {
        Stack.push(Queue.front());
        Queue.pop();
    }
    while (!Stack.empty()) {
        Queue.push(Stack.top());
        Stack.pop();
    }
}

// Driver code
int main()
{
    queue<int> Queue;
    Queue.push(10);
    Queue.push(20);
    Queue.push(30);
    Queue.push(40);
    Queue.push(50);
    Queue.push(60);
    Queue.push(70);
    Queue.push(80);
    Queue.push(90);
    Queue.push(100);

    reverseQueue(Queue);
    Print(Queue);
}

Java

// Java program to reverse a Queue
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;

// Java program to reverse a queue
public class Queue_reverse {

    static Queue<Integer> queue;

    // Utility function to print the queue
    static void Print()
    {
        while (!queue.isEmpty()) {
            System.out.print(queue.peek() + ", ");
            queue.remove();
        }
    }

    // Function to reverse the queue
    static void reversequeue()
    {
        Stack<Integer> stack = new Stack<>();
        while (!queue.isEmpty()) {
            stack.add(queue.peek());
            queue.remove();
        }
        while (!stack.isEmpty()) {
            queue.add(stack.peek());
            stack.pop();
        }
    }

    // Driver code
    public static void main(String args[])
    {
        queue = new LinkedList<Integer>();
        queue.add(10);
        queue.add(20);
        queue.add(30);
        queue.add(40);
        queue.add(50);
        queue.add(60);
        queue.add(70);
        queue.add(80);
        queue.add(90);
        queue.add(100);

        reversequeue();
        Print();
    }
}
// This code is contributed by Sumit Ghosh

Python3

# Python3 program to reverse a queue
from collections import deque

# Function to reverse the queue


def reversequeue(queue):
    Stack = []

    while (queue):
        Stack.append(queue[0])
        queue.popleft()

    while (len(Stack) != 0):
        queue.append(Stack[-1])
        Stack.pop()


# Driver code
if __name__ == '__main__':
    queue = deque([10, 20, 30, 40, 50, 60, 70, 80, 90, 100])

    reversequeue(queue)
    print(queue)

# This code is contributed by PranchalK

C#

// c# program to reverse a Queue
using System;
using System.Collections.Generic;

public class GFG {

    public static LinkedList<int> queue;

    // Utility function to print the queue
    public static void Print()
    {
        while (queue.Count > 0) {
            Console.Write(queue.First.Value + ", ");
            queue.RemoveFirst();
        }
    }

    // Function to reverse the queue
    public static void reversequeue()
    {
        Stack<int> stack = new Stack<int>();
        while (queue.Count > 0) {
            stack.Push(queue.First.Value);
            queue.RemoveFirst();
        }
        while (stack.Count > 0) {
            queue.AddLast(stack.Peek());
            stack.Pop();
        }
    }

    // Driver code
    public static void Main(string[] args)
    {
        queue = new LinkedList<int>();
        queue.AddLast(10);
        queue.AddLast(20);
        queue.AddLast(30);
        queue.AddLast(40);
        queue.AddLast(50);
        queue.AddLast(60);
        queue.AddLast(70);
        queue.AddLast(80);
        queue.AddLast(90);
        queue.AddLast(100);

        reversequeue();
        Print();
    }
}

// This code is contributed by Shrikant13

Javascript

<script>
    // Javascript program to reverse a Queue

    let queue = [];

    // Utility function to print the queue
    function Print()
    {
        while (queue.length > 0) {
            document.write( queue[0] + ", ");
            queue.shift();
        }
    }

    // Function to reverse the queue
    function reversequeue()
    {
        let stack = [];
        while (queue.length > 0) {
            stack.push(queue[0]);
            queue.shift();
        }
        while (stack.length > 0) {
            queue.push(stack[stack.length - 1]);
            stack.pop();
        }
    }

    queue = []
    queue.push(10);
    queue.push(20);
    queue.push(30);
    queue.push(40);
    queue.push(50);
    queue.push(60);
    queue.push(70);
    queue.push(80);
    queue.push(90);
    queue.push(100);

    reversequeue();
    Print();

</script>

Output

100 90 80 70 60 50 40 30 20 10 

Time Complexity: O(N), As we need to insert all the elements in the stack and later to the queue.
Auxiliary Space: O(N), Use of stack to store values. 

Reversing a Queue using recursion: 

Instead of explicitly using stack goal can be achieved using recursion (recursion at backend will itself maintain stack).

Follow the below steps to implement the idea:

• Recursively perform the following steps:
  • If the queue size is 0 return.
  • Else pop and store the front element and recur for remaining queue.
  • push the current element in the queue.

Thank you Nakshatra Chhillar for suggesting this approach and contributing the code 

Below is the implementation of above approach:

C++

// CPP program to reverse a Queue
#include <bits/stdc++.h>
using namespace std;

// Utility function to print the queue
void Print(queue<int>& Queue)
{
    while (!Queue.empty()) {
        cout << Queue.front() << " ";
        Queue.pop();
    }
}

// Function to reverse the queue
void reverseQueue(queue<int>& q)
{
    // base case
    if (q.size() == 0)
        return;
    // storing front(first element) of queue
    int fr = q.front();

    // removing front
    q.pop();

    // asking recursion to reverse the
    // leftover queue
    reverseQueue(q);

    // placing first element
    // at its correct position
    q.push(fr);
}

// Driver code
int main()
{
    queue<int> Queue;
    Queue.push(10);
    Queue.push(20);
    Queue.push(30);
    Queue.push(40);
    Queue.push(50);
    Queue.push(60);
    Queue.push(70);
    Queue.push(80);
    Queue.push(90);
    Queue.push(100);

    reverseQueue(Queue);
    Print(Queue);
}
// This code is contributed by Nakshatra Chhillar

Output

100 90 80 70 60 50 40 30 20 10 

Time Complexity: O(N). 
Auxiliary Space: O(N). The recursion stack contains all elements of queue at a moment. 

This article is contributed by Raghav Sharma and improved by Nakshatra Chhillar . If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.