Skip to content
Related Articles
Open in App
Not now

Related Articles

Reversing the first K elements of a Queue

Improve Article
Save Article
  • Difficulty Level : Easy
  • Last Updated : 13 Jan, 2023
Improve Article
Save Article

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 <bits/stdc++.h>
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<Integer> reverseFirstK(Queue<Integer> 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<Integer> 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<Integer> 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;
       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[0];
  }
 
  // 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 <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);
            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




<script>
 
// JavaScript program to reverse first
// k elements of a queue.
 
/* Function to reverse the first
K elements of the Queue */
function reverseQueueFirstKElements(k,Queue)
{
    if (Queue.length == 0 || k > Queue.length)
        return;
    if (k <= 0)
        return;
 
    let Stack = [];
 
    /* Push the first K elements
    into a Stack*/
    for (let i = 0; i < k; i++) {
        Stack.push(Queue.shift());
    }
 
    /* Enqueue the contents of stack
    at the back of the queue*/
    while (Stack.length > 0) {
        Queue.push(Stack.pop());
    }
 
    /* Remove the remaining elements and
    enqueue them at the end of the Queue*/
    for (let i = 0; i < Queue.length - k; i++) {
        Queue.push(Queue.shift());
    }
}
 
/* Utility Function to print the Queue */
function Print(Queue)
{
    while (Queue.length > 0) {
        document.write(Queue.shift()," ");
    }
}
 
// Driver code
 
let 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;
reverseQueueFirstKElements(k, Queue);
Print(Queue);
 
// This code is contributed by shinjanpatra
 
</script>


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.


My Personal Notes arrow_drop_up
Related Articles

Start Your Coding Journey Now!