Tutorialspoint.dev

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]



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.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 [tabby title="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

Output:

 50 40 30 20 10 60 70 80 90 100

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

leave a comment

code

0 Comments

load comments

Subscribe to Our Newsletter