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Reversing a queue using recursion

  • Difficulty Level : Easy
  • Last Updated : 24 Jun, 2021

Given a queue, write a recursive function to reverse it. 
Standard operations allowed : 
enqueue(x) : Add an item x to rear of queue. 
dequeue() : Remove an item from front of queue. 
empty() : Checks if a queue is empty or not.
Examples : 
 

Input : Q = [5, 24, 9, 6, 8, 4, 1, 8, 3, 6]
Output : Q = [6, 3, 8, 1, 4, 8, 6, 9, 24, 5]

Explanation : Output queue is the reverse of the input queue.

Input : Q = [8, 7, 2, 5, 1]
Output : Q = [1, 5, 2, 7, 8]

Recursive Algorithm : 
 

  1. The pop element from the queue if the queue has elements otherwise return empty queue.
  2. Call reverseQueue function for the remaining queue.
  3. Push the popped element in the resultant reversed queue.

 

Pseudo Code : 
 



queue reverseFunction(queue)
{
    if (queue is empty)
       return queue;
    else {
       data = queue.front()
       queue.pop()
       queue = reverseFunction(queue);
       q.push(data);
       return queue;
    }
}

 

 

C++




// C++ code for reversing a queue
#include <bits/stdc++.h>
using namespace std;
 
// Utility function to print the queue
void printQueue(queue<long long int> Queue)
{
    while (!Queue.empty()) {
        cout << Queue.front() << " ";
        Queue.pop();
    }
}
 
// Recursive function to reverse the queue
void reverseQueue(queue<long long int>& q)
{
    // Base case
    if (q.empty())
        return;
 
    // Dequeue current item (from front) 
    long long int data = q.front();
    q.pop();
 
    // Reverse remaining queue 
    reverseQueue(q);
 
    // Enqueue current item (to rear) 
    q.push(data);
}
 
// Driver code
int main()
{
    queue<long long int> Queue;
    Queue.push(56);
    Queue.push(27);
    Queue.push(30);
    Queue.push(45);
    Queue.push(85);
    Queue.push(92);
    Queue.push(58);
    Queue.push(80);
    Queue.push(90);
    Queue.push(100);
    reverseQueue(Queue);
    printQueue(Queue);
}

Java




// Java program to reverse a Queue by recursion
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
  
// Java program to reverse a queue recursively
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();
        }
    }
  
// Recurrsive function to reverse the queue
static Queue<Integer> reverseQueue(Queue<Integer> q)
{
    // Base case
    if (q.isEmpty())
        return q;
 
    // Dequeue current item (from front) 
    int data = q.peek();
    q.remove();
 
    // Reverse remaining queue 
    q = reverseQueue(q);
 
    // Enqueue current item (to rear) 
    q.add(data);
         
    return q;
}
  
// Driver code
public static void main(String args[])
{
    queue = new LinkedList<Integer>();
    queue.add(56);
    queue.add(27);
    queue.add(30);
    queue.add(45);
    queue.add(85);
    queue.add(92);
    queue.add(58);
    queue.add(80);
    queue.add(90);
    queue.add(100);
    queue = reverseQueue(queue);
    Print();
}
}

Python3




from queue import Queue
 
 
def reverse_queue(queue: Queue):
    # Base case
    if queue.empty():
        return
 
    # Dequeue current item (from front)
    item = queue.queue[0]
    queue.get()
 
    # Reverse remaining queue
    reverse_queue(queue)
 
    # Enqueue current item (to rear)
    queue.put(item)
 
 
def print_queue(queue: Queue):
    while not queue.empty():
        print(queue.queue[0], end=" ")
        queue.get()
    print()
 
 
# Driver Code
if __name__ == "__main__":
    q = Queue()
    q.put(56)
    q.put(27)
    q.put(30)
    q.put(45)
    q.put(85)
    q.put(92)
    q.put(58)
    q.put(80)
    q.put(90)
    q.put(100)
 
    reverse_queue(q)
    print_queue(q)

C#




// C# code for reversing a queue
using System;
using System.Collections.Generic;
 
class GFG
{
    // Utility function
    // to print the queue
    static void printQueue(Queue<long> queue)
    {
        while (queue.Count != 0)
        {
            Console.Write(queue.Peek() + " ");
            queue.Dequeue();
        }
    }
     
    // Recursive function
    // to reverse the queue
    static void reverseQueue(ref Queue<long> q)
    {
        // Base case
        if (q.Count == 0)
            return;
     
        // Dequeue current
        // item (from front)
        long data = q.Peek();
        q.Dequeue();
     
        // Reverse remaining queue
        reverseQueue(ref q);
     
        // Enqueue current
        // item (to rear)
        q.Enqueue(data);
    }    
     
    // Driver code
    static void Main()
    {
        Queue<long> queue = new Queue<long>();
        queue.Enqueue(56);
        queue.Enqueue(27);
        queue.Enqueue(30);
        queue.Enqueue(45);
        queue.Enqueue(85);
        queue.Enqueue(92);
        queue.Enqueue(58);
        queue.Enqueue(80);
        queue.Enqueue(90);
        queue.Enqueue(100);
        reverseQueue(ref queue);
        printQueue(queue);
    }
}
 
// This code is contributed by
// Manish Shaw(manishshaw1)
Output: 
100 90 80 58 92 85 45 30 27 56

 

Time Complexity : O(n). 
Video: 
 

 

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