# Reversing a Queue

• Difficulty Level : Easy
• Last Updated : 20 Jul, 2022

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 1 (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 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)

Implementation:

## C++

 `// CPP program to reverse a Queue``#include ``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 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 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();``        ``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

 ``

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.

Approach-2 (using recursion) : Instead of explicitly using stack we can mimic it using recursion ( recursion at backend will itself maintain stack).Lets have a look at the code .

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

Implementation:

## C++

 `// CPP program to reverse a Queue``#include ``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). 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.

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