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Reversing the first K elements of a Queue
• Difficulty Level : Easy
• Last Updated : 20 Jan, 2021

Given an integer k and a queue of integers, we need to reverse the order of the first k elements of the queue, leaving the other elements in the same relative order.
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
• size() : Returns number of elements in queue.
• front() : Finds front item.

Examples:

```Input : Q = [10, 20, 30, 40, 50, 60,
70, 80, 90, 100]
k = 5
Output : Q = [50, 40, 30, 20, 10, 60,
70, 80, 90, 100]

Input : Q = [10, 20, 30, 40, 50, 60,
70, 80, 90, 100]
k = 4
Output : Q = [40, 30, 20, 10, 50, 60,
70, 80, 90, 100]```

The idea is to use an auxiliary stack

1. Create an empty stack.
2. One by one dequeue first K items from given queue and push the dequeued items to stack.
3. Enqueue the contents of stack at the back of the queue
4. Dequeue (size-k) elements from the front and enque them one by one to the same queue.

## C++

 `// C++ program to reverse first``// k elements of a queue.``#include ``using` `namespace` `std;` `/* Function to reverse the first``   ``K elements of the Queue */``void` `reverseQueueFirstKElements(``    ``int` `k, queue<``int``>& Queue)``{``    ``if` `(Queue.empty() == ``true``        ``|| k > Queue.size())``        ``return``;``    ``if` `(k <= 0)``        ``return``;` `    ``stack<``int``> Stack;` `    ``/* Push the first K elements``       ``into a Stack*/``    ``for` `(``int` `i = 0; i < k; i++) {``        ``Stack.push(Queue.front());``        ``Queue.pop();``    ``}` `    ``/* Enqueue the contents of stack``       ``at the back of the queue*/``    ``while` `(!Stack.empty()) {``        ``Queue.push(Stack.top());``        ``Stack.pop();``    ``}` `    ``/* Remove the remaining elements and``       ``enqueue them at the end of the Queue*/``    ``for` `(``int` `i = 0; i < Queue.size() - k; i++) {``        ``Queue.push(Queue.front());``        ``Queue.pop();``    ``}``}` `/* Utility Function to print the Queue */``void` `Print(queue<``int``>& Queue)``{``    ``while` `(!Queue.empty()) {``        ``cout << Queue.front() << ``" "``;``        ``Queue.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);` `    ``int` `k = 5;``    ``reverseQueueFirstKElements(k, Queue);``    ``Print(Queue);``}`

## Java

 `// Java program to reverse first k elements``// of a queue.``import` `java.util.LinkedList;``import` `java.util.Queue;``import` `java.util.Stack;` `public` `class` `Reverse_k_element_queue {` `    ``static` `Queue queue;` `    ``// Function to reverse the first``    ``// K elements of the Queue``    ``static` `void` `reverseQueueFirstKElements(``int` `k)``    ``{``        ``if` `(queue.isEmpty() == ``true``            ``|| k > queue.size())``            ``return``;``        ``if` `(k <= ``0``)``            ``return``;` `        ``Stack stack = ``new` `Stack();` `        ``// Push the first K elements into a Stack``        ``for` `(``int` `i = ``0``; i < k; i++) {``            ``stack.push(queue.peek());``            ``queue.remove();``        ``}` `        ``// Enqueue the contents of stack``        ``// at the back of the queue``        ``while` `(!stack.empty()) {``            ``queue.add(stack.peek());``            ``stack.pop();``        ``}` `        ``// Remove the remaining elements and enqueue``        ``// them at the end of the Queue``        ``for` `(``int` `i = ``0``; i < queue.size() - k; i++) {``            ``queue.add(queue.peek());``            ``queue.remove();``        ``}``    ``}` `    ``// Utility Function to print the Queue``    ``static` `void` `Print()``    ``{``        ``while` `(!queue.isEmpty()) {``            ``System.out.print(queue.peek() + ``" "``);``            ``queue.remove();``        ``}``    ``}` `    ``// 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``);` `        ``int` `k = ``5``;``        ``reverseQueueFirstKElements(k);``        ``Print();``    ``}``}``// This code is contributed by Sumit Ghosh`

## Python3

 `# Python3 program to reverse first k``# elements of a queue.``from` `queue ``import` `Queue` `# Function to reverse the first K``# elements of the Queue``def` `reverseQueueFirstKElements(k, Queue):``    ``if` `(Queue.empty() ``=``=` `True` `or``             ``k > Queue.qsize()):``        ``return``    ``if` `(k <``=` `0``):``        ``return` `    ``Stack ``=` `[]` `    ``# put the first K elements``    ``# into a Stack``    ``for` `i ``in` `range``(k):``        ``Stack.append(Queue.queue[``0``])``        ``Queue.get()` `    ``# Enqueue the contents of stack``    ``# at the back of the queue``    ``while` `(``len``(Stack) !``=` `0` `):``        ``Queue.put(Stack[``-``1``])``        ``Stack.pop()` `    ``# Remove the remaining elements and``    ``# enqueue them at the end of the Queue``    ``for` `i ``in` `range``(Queue.qsize() ``-` `k):``        ``Queue.put(Queue.queue[``0``])``        ``Queue.get()` `# Utility Function to print the Queue``def` `Print``(Queue):``    ``while` `(``not` `Queue.empty()):``        ``print``(Queue.queue[``0``], end ``=``" "``)``        ``Queue.get()` `# 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``)` `    ``k ``=` `5``    ``reverseQueueFirstKElements(k, Queue)``    ``Print``(Queue)` `# This code is contributed by PranchalK`

## C#

 `// C# program to reverse first k elements``// of a queue.``using` `System;``using` `System.Collections.Generic;` `class` `GFG {` `    ``public` `static` `LinkedList<``int``> queue;` `    ``// Function to reverse the first K``    ``// elements of the Queue``    ``public` `static` `void` `reverseQueueFirstKElements(``int` `k)``    ``{``        ``if` `(queue.Count == 0 || k > queue.Count) {``            ``return``;``        ``}``        ``if` `(k <= 0) {``            ``return``;``        ``}` `        ``Stack<``int``> stack = ``new` `Stack<``int``>();` `        ``// Push the first K elements into a Stack``        ``for` `(``int` `i = 0; i < k; i++) {``            ``stack.Push(queue.First.Value);``            ``queue.RemoveFirst();``        ``}` `        ``// Enqueue the contents of stack at``        ``// the back of the queue``        ``while` `(stack.Count > 0) {``            ``queue.AddLast(stack.Peek());``            ``stack.Pop();``        ``}` `        ``// Remove the remaining elements and``        ``// enqueue them at the end of the Queue``        ``for` `(``int` `i = 0; i < queue.Count - k; i++) {``            ``queue.AddLast(queue.First.Value);
• Complexity Analysis:``Time Complexity: O(n3).``As three nested ``for` `loops are used.``Auxiliary Space :No use of any data structure ``for` `storing values-: O(1)``
• ``        ``queue.RemoveFirst();``        ``}``    ``}` `    ``// Utility Function to print the Queue``    ``public` `static` `void` `Print()``    ``{``        ``while` `(queue.Count > 0) {``            ``Console.Write(queue.First.Value + ``" "``);``            ``queue.RemoveFirst();``        ``}``    ``}` `    ``// 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);` `        ``int` `k = 5;``        ``reverseQueueFirstKElements(k);``        ``Print();``    ``}``}` `// This code is contributed by Shrikant13`
Output:
`50 40 30 20 10 60 70 80 90 100`

Complexity Analysis:

• Time Complexity: O(n+k).
Where ‘n’ is the total number of elements in the queue and ‘k’ is the number of elements to be reversed. This is because firstly the whole queue is emptied into the stack and after that first ‘k’ elements are emptied and enqueued in the same way.
• Auxiliary Space :Use of stack to store values for the purpose of reversing-: O(n)

-s
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