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# Reversing the first K elements of a Queue

• Difficulty Level : Easy
• Last Updated : 13 Jan, 2023

Given an integer k and a queue of integers, The task is 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.

Approach:

We can use recursive call stack and we can add remaining items of front without using additional queue.

Below are the steps:

1. Reverse first k elements.

2. Remove from front and add to back (N – K) elements.

Below is the implementation of above approach:

## C++

 `#include ` `using` `namespace` `std;`   `void` `solve(queue<``int``>& q, ``int` `k);`   `// Function to reverse first k elements of a queue` `queue<``int``> reverseFirstK(queue<``int``> q, ``int` `k) {` `    ``solve(q, k);` `    ``int` `s = q.size() - k;` `    ``while` `(s-- > 0) {` `        ``int` `x = q.front();` `        ``q.pop();` `        ``q.push(x);` `    ``}` `    ``return` `q;` `}`   `void` `solve(queue<``int``>& q, ``int` `k) {` `    ``if` `(k == 0) ``return``;` `    ``int` `e = q.front();` `    ``q.pop();` `    ``solve(q, k - 1);` `    ``q.push(e);` `}`   `// 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;` `    ``queue = reverseFirstK(queue, k);`   `    ``// Printing queue` `    ``while` `(!queue.empty()) {` `        ``cout << queue.front() << ``" "``;` `        ``queue.pop();` `    ``}` `    ``return` `0;` `}`

## Java

 `import` `java.io.*;` `import` `java.util.*;`   `public` `class` `GFG {  ` `  ``// Function to reverse first k elements of a queue.` `   ``static` `Queue reverseFirstK(Queue q, ``int` `k) {` `       ``solve(q, k);` `       ``int` `s = q.size() - k;` `       ``while``( s-- > ``0``){` `           ``int` `x = q.poll();` `           ``q.add(x);` `       ``}` `       ``return` `q;` `   ``}` `   ``static` `void` `solve(Queue q, ``int` `k){` `       ``if``(k == ``0``) ``return``;` `       ``int` `e = q.poll();` `       ``solve(q, k - ``1``);` `       ``q.add(e);` `   ``}` ` `  `   ``// driver code` `   ``public` `static` `void` `main (String[] args) {` `       ``Queue 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``;` `       ``queue = reverseFirstK(queue, k);` `       ``// printing queue` `      ``while` `(!queue.isEmpty()) {` `           ``System.out.print(queue.poll() + ``" "``);` `       ``}` `   ``}` `}`

## Python3

 `from` `collections ``import` `deque`   `def` `reverse_first_k(q, k):` `    ``solve(q, k)` `    ``s ``=` `len``(q) ``-` `k` `    ``for` `_ ``in` `range``(s):` `        ``x ``=` `q.popleft()` `        ``q.append(x)` `    ``return` `q`   `def` `solve(q, k):` `    ``if` `k ``=``=` `0``:` `        ``return` `    ``e ``=` `q.popleft()` `    ``solve(q, k ``-` `1``)` `    ``q.append(e)`   `# Driver code` `queue ``=` `deque([``10``, ``20``, ``30``, ``40``, ``50``, ``60``, ``70``, ``80``, ``90``, ``100``])` `k ``=` `5` `queue ``=` `reverse_first_k(queue, k)`   `# Printing queue` `while` `queue:` `    ``print``(queue.popleft(), end``=``' '``)`

## Javascript

 `// JavaScript Code to implement queue data structure in Javascript` `class Queue {` `  ``constructor() {` `    ``this``.items = [];` `  ``}`   `  ``// add element to the queue` `  ``push(element) {` `    ``return` `this``.items.push(element);` `  ``}`   `  ``// remove element from the queue` `  ``pop() {` `    ``if` `(``this``.items.length > 0) {` `      ``return` `this``.items.shift();` `    ``}` `  ``}`   `  ``// view the first element` `  ``front() {` `    ``return` `this``.items;` `  ``}`   `  ``// check if the queue is empty` `  ``isEmpty() {` `    ``return` `this``.items.length == 0;` `  ``}`   `  ``// the size of the queue` `  ``size() {` `    ``return` `this``.items.length;` `  ``}` `}`   `// Function to reverse first k elements of a queue` `function` `reverseFirstK(queue, k) {` `  ``solve(queue, k);` `  ``let s = queue.size() - k;` `  ``while` `(s-- > 0) {` `    ``let x = queue.front();` `    ``queue.pop();` `    ``queue.push(x);` `  ``}` `  ``return` `queue;` `}`   `function` `solve(queue, k) {` `  ``if` `(k == 0) ``return``;` `  ``let e = queue.front();` `  ``queue.pop();` `  ``solve(queue, k - 1);` `  ``queue.push(e);` `}`   `// Driver code`   `let queue = ``new` `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);`   `let k = 5;` `q = reverseFirstK(queue, k);`   `// Printing queue` `while` `(!q.isEmpty()) {` `  ``console.log(q.front());` `  ``q.pop();` `}`   `// This code is contributed by satwiksuman.`

Output

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

Approach:

The idea is to use an auxiliary stack. Store the first k elements of the queue in a stack and pop it from the queue, then push it back to the queue and perform pop operation for n-k times and again push the popped element.

Follow the below steps to implement the idea:

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

Below is the implementation of above approach:

## 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);` `            ``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`

## Javascript

 ``

Output

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

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: O(k) where k is no of elements to be reversed since stack is being used to store values for the purpose of reversing.

This article is contributed by Raghav Sharma. 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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