Tutorialspoint.dev

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]



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 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

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


Output:

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

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



This article is attributed to GeeksforGeeks.org

You Might Also Like

leave a comment

code

0 Comments

load comments

Subscribe to Our Newsletter