Give an algorithm for reversing a queue Q. Only the following standard operations are allowed on queue.
- enqueue(x): Add an item x to the rear of the queue.
- dequeue(): Remove an item from the front of the queue.
- empty(): Checks if a queue is empty or not.
The task is to reverse the queue.
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]
Reversing a Queue 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 a data structure that 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.
Follow the below steps to implement the idea:
- Pop the elements from the queue and insert into the stack now topmost element of the stack is the last element of the queue.
- 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.
Below is the implementation of above approach:
C++
// CPP program to reverse a Queue#include <bits/stdc++.h>using namespace std;// Utility function to print the queuevoid Print(queue<int>& Queue){ while (!Queue.empty()) { cout << Queue.front() << " "; Queue.pop(); }}// Function to reverse the queuevoid 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 codeint 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 Queueimport java.util.LinkedList;import java.util.Queue;import java.util.Stack;// Java program to reverse a queuepublic 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 queuefrom collections import deque# Function to reverse the queuedef reversequeue(queue): Stack = [] while (queue): Stack.append(queue[0]) queue.popleft() while (len(Stack) != 0): queue.append(Stack[-1]) Stack.pop()# Driver codeif __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 Queueusing 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
<script> // Javascript program to reverse a Queue let queue = []; // Utility function to print the queue function Print() { while (queue.length > 0) { document.write( queue[0] + ", "); queue.shift(); } } // Function to reverse the queue function reversequeue() { let stack = []; while (queue.length > 0) { stack.push(queue[0]); queue.shift(); } while (stack.length > 0) { queue.push(stack[stack.length - 1]); stack.pop(); } } 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(); Print(); </script> |
Output
100 90 80 70 60 50 40 30 20 10
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.
Reversing a Queue using recursion:
Instead of explicitly using stack goal can be achieved using recursion (recursion at backend will itself maintain stack).
Follow the below steps to implement the idea:
- Recursively perform the following steps:
- If the queue size is 0 return.
- Else pop and store the front element and recur for remaining queue.
- push the current element in the queue.
Thank you Nakshatra Chhillar for suggesting this approach and contributing the code
Below is the implementation of above approach:
C++
// CPP program to reverse a Queue#include <bits/stdc++.h>using namespace std;// Utility function to print the queuevoid Print(queue<int>& Queue){ while (!Queue.empty()) { cout << Queue.front() << " "; Queue.pop(); }}// Function to reverse the queuevoid 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 codeint 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 |
Java
/*package whatever //do not write package name here */import java.io.*;import java.util.*;class GFG { // Utility function to print the queue public static void Print(Queue<Integer> Queue) { while (Queue.size() > 0) { System.out.print(Queue.peek() + " "); Queue.remove(); } } // Function to reverse the queue public static void reverseQueue(Queue<Integer> q) { // base case if (q.size() == 0) return; // storing front(first element) of queue int fr = q.peek(); // removing front q.remove(); // asking recursion to reverse the // leftover queue reverseQueue(q); // placing first element // at its correct position q.add(fr); } public static void main(String[] args) { Queue<Integer> 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(Queue); Print(Queue); }}// This code is contributed by akashish__ |
Python3
# Python3 program to reverse a Queue# Utility function to print the queuedef Print(Queue): while (len(Queue) > 0): print(Queue[0],end = " ") Queue.pop(0)# Function to reverse the queuedef reverseQueue(q): # base case if (len(q) == 0): return # storing front(first element) of queue fr = q[0] # removing front q.pop(0) # asking recursion to reverse the # leftover queue reverseQueue(q) # placing first element # at its correct position q.append(fr)# Driver codeQueue = []Queue.append(10)Queue.append(20)Queue.append(30)Queue.append(40)Queue.append(50)Queue.append(60)Queue.append(70)Queue.append(80)Queue.append(90)Queue.append(100)reverseQueue(Queue)Print(Queue)# This code is contributed by akashish__ |
C#
// CPP program to reverse a Queueusing System;using System.Collections;public class GFG { // Utility function to print the queue public static void Print(Queue Queue) { while (Queue.Count > 0) { Console.Write(Queue.Peek()); Console.Write(" "); Queue.Dequeue(); } } // Function to reverse the queue public static void reverseQueue(Queue q) { // base case if (q.Count == 0) return; // storing front(first element) of queue int fr = (int)q.Peek(); // removing front q.Dequeue(); // asking recursion to reverse the // leftover queue reverseQueue(q); // placing first element // at its correct position q.Enqueue(fr); } // Driver code static public void Main() { Queue Queue = new Queue(); Queue.Enqueue(10); Queue.Enqueue(20); Queue.Enqueue(30); Queue.Enqueue(40); Queue.Enqueue(50); Queue.Enqueue(60); Queue.Enqueue(70); Queue.Enqueue(80); Queue.Enqueue(90); Queue.Enqueue(100); reverseQueue(Queue); Print(Queue); }} |
Javascript
// JS program to reverse a Queue// Utility function to print the queuefunction Print(Queue) { while (Queue.length != 0) { console.log(Queue[0]); Queue.shift(); }}// Function to reverse the queuefunction reverseQueue(q) { // base case if (q.length == 0) return; // storing front(first element) of queue let fr = q[0]; // removing front q.shift(); // asking recursion to reverse the // leftover queue reverseQueue(q); // placing first element // at its correct position q.push(fr);}// Driver codelet 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 adityamaharshi21. |
Output
100 90 80 70 60 50 40 30 20 10
Time Complexity: O(N).
Auxiliary Space: O(N). The 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 neveropen 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 neveropen main page and help other Geeks.
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