Reversing a Queue

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

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

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]

Approach: 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 data-structure which 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. The stack could help in approaching this problem. This will be a two-step process:

  1. Pop the elements from the queue and insert into the stack. (Topmost element of the stack is the last element of the queue)
  2. 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)

C++

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// 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);
}

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Java

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// 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

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Python3

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# Python3 program to reverse a queue 
from queue import Queue 
  
# Utility function to print the queue 
def Print(queue):
    while (not queue.empty()):
        print(queue.queue[0], end = ", "
        queue.get()
  
# Function to reverse the queue 
def reversequeue(queue):
    Stack = [] 
    while (not queue.empty()): 
        Stack.append(queue.queue[0]) 
        queue.get()
    while (len(Stack) != 0): 
        queue.put(Stack[-1]) 
        Stack.pop()
  
# Driver code 
if __name__ == '__main__':
    queue = Queue()
    queue.put(10
    queue.put(20
    queue.put(30
    queue.put(40
    queue.put(50
    queue.put(60
    queue.put(70
    queue.put(80
    queue.put(90
    queue.put(100
  
    reversequeue(queue) 
    Print(queue)
  
# This code is contributed by PranchalK

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C#

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// 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

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Output:



100, 90, 80, 70, 60, 50, 40, 30, 20, 10

Complexity Analysis:

  • 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.

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

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