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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 <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.LinkedList;
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()) {
            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<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);
 
        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);<li><strong>Complexity Analysis:</strong>
<strong>Time Complexity:</strong> O(n<sup>3</sup>).
As three nested for loops are used.
<strong>Auxiliary Space :</strong>No use of any data structure for storing values-: O(1)
</li>
        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 
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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