# Reversing the first K elements of a Queue

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]
```

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

The idea is to use an auxiliary stack.

1) Create an empty stack.
2) One by one dequeue items from given queue and push the dequeued items to stack.
3) Enqueue the contents of stack at the back of the queue
4) Reverse the whole queue.

## C++

```// C++ program to reverse first k elements of a queue.
#include <bits/stdc++.h>
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.Queue;
import java.util.Stack;

public class Reverse_k_element_queue {

static Queue<Integer> 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<Integer> stack = new Stack<Integer>();

// 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()) {
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.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[]) {

int k = 5;
reverseQueueFirstKElements(k);
Print();
}
}
// This code is contributed by Sumit Ghosh
```

Output:

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

This article is contributed by Raghav Sharma. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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