Reversing a Queue - GeeksforGeeks

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

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 = [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]

Recommended: Please solve it on PRACTICE first, before moving on to the solution.

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

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My Personal Notes

Recommended: Please solve it on PRACTICE first, before moving on to the solution.

C++

Java

Python3

C#

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