Reversing the first K elements of a Queue

Last Updated : 13 Apr, 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 <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=' ')

C#

using System;
using System.Collections.Generic;
 
class GFG {
     
    public static LinkedList<int> queue;
     
    public static void solve(int k) {
        if (k == 0){
            return;
        }
         
        int e = queue.First.Value;
        queue.RemoveFirst();
        solve(k - 1);
        queue.AddLast(e);
    } 
     
    // Function to reverse first k elements of a queue
    public static void reverseFirstK(int k) {
        solve(k);
        int s = queue.Count - k;
        while (s > 0) {
            int x = queue.First.Value;
            queue.RemoveFirst();
            queue.AddLast(x);
            s = s - 1;
        }
    }
     
    // 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;
        reverseFirstK(k);
 
        // Printing queue
        while (queue.Count > 0)
        {
            Console.Write(queue.First.Value + " ");
            queue.RemoveFirst();
        }
    }
}
 
// The code is contributed by Gautam goel.

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

Time and Space complexity:

The time complexity of the given program can be analyzed as follows:

The function reverseFirstK calls the recursive function solve, which takes O(k) time to reverse the first k elements of the queue.
The remaining part of the function reverseFirstK takes O(n-k) time to move the remaining elements to the end of the queue.
The overall time complexity of the function reverseFirstK is O(n), where n is the size of the input queue.
Therefore, the time complexity of the entire program is O(n).

The space complexity of the program is also O(n), as the input queue is stored in memory along with some additional variables used in the program, such as the integer variable s. However, the space used by the recursive function solve is O(k), as it calls itself recursively k times, where k is the number of elements to be reversed.

Therefore, the overall space complexity of the program is O(n+k).

Recommended PracticeReverse First K elements of QueueTry It!

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.

Using Doubly Ended Queue:

$The idea is to use the deque as a temporary container to store the first k elements of the queue in reverse order.$

Follow the steps below to implement the above idea:

Dequeue the first k elements of the queue and push them onto a deque using the push_front() function.
Pop the elements from the deque one by one using the pop_front() function and enqueue them back into the queue using the push() function.
Dequeue the remaining elements from the queue and enqueue them back into the queue using the push() and pop() functions.

Below is the implementation of the above approach:

C++

// C++ program to reverse first
// k elements of a queue using dequeue.
#include <deque>
#include <iostream>
#include <queue>
 
using namespace std;
// Function to reverse first k element of the queue
void reverseFirstK(queue<int>& q, int k)
{
    deque<int> d;
 
    // Dequeue the first k elements of the queue and push
    // them onto a deque
    for (int i = 0; i < k; i++) {
        d.push_front(q.front());
        q.pop();
    }
 
    // Pop the elements from the deque and enqueue them back
    // into the queue
    while (!d.empty()) {
        q.push(d.front());
        d.pop_front();
    }
 
    // Dequeue the remaining elements from the queue and
    // enqueue them back into the queue
    for (int i = 0; i < q.size() - k; i++) {
        q.push(q.front());
        q.pop();
    }
}
// Driver code.
int main()
{
    queue<int> q;
    q.push(10);
    q.push(20);
    q.push(30);
    q.push(40);
    q.push(50);
    q.push(60);
    q.push(70);
    q.push(80);
    q.push(90);
    q.push(100);
 
    int k = 5;
 
    // function call.
    reverseFirstK(q, k);
 
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    return 0;
}
// This code is contributed by Veerendra_Singh_Rajpoot

Java

import java.util.*;
 
public class ReverseFirstKQueue {
    // Function to reverse first k elements of the queue
    public static void reverseFirstK(Queue<Integer> q, int k){
        Deque<Integer> d = new ArrayDeque<>();
 
        // Dequeue the first k elements of the queue and
        // push them onto a deque
        for (int i = 0; i < k; i++) {
            d.push(q.poll());
        }
 
        // Pop the elements from the deque and enqueue them
        // back into the queue
        while (!d.isEmpty()) {
            q.add(d.pop());
        }
 
        // Dequeue the remaining elements from the queue and
        // enqueue them back into the queue
        for (int i = 0; i < q.size() - k; i++) {
            q.add(q.poll());
        }
    }
 
    // Driver code.
    public static void main(String[] args){
        Queue<Integer> q = new LinkedList<Integer>();
        q.add(10);
        q.add(20);
        q.add(30);
        q.add(40);
        q.add(50);
        q.add(60);
        q.add(70);
        q.add(80);
        q.add(90);
        q.add(100);
 
        int k = 5;
 
        // function call.
        reverseFirstK(q, k);
 
        while (!q.isEmpty()) {
            System.out.print(q.peek() + " ");
            q.poll();
        }
    }
}

Python3

# Python program to reverse first k elements of a queue using dequeue.
from collections import deque
 
def reverseFirstK(q, k):
    d = deque()
 
    # Dequeue the first k elements of the queue and push
    # them onto a deque
    for i in range(k):
        d.appendleft(q.popleft())
 
    # Pop the elements from the deque and enqueue them back
    # into the queue
    while d:
        q.append(d.popleft())
 
    # Dequeue the remaining elements from the queue and
    # enqueue them back into the queue
    for i in range(len(q) - k):
        q.append(q.popleft())
 
# Driver code.
q = deque()
q.append(10)
q.append(20)
q.append(30)
q.append(40)
q.append(50)
q.append(60)
q.append(70)
q.append(80)
q.append(90)
q.append(100)
 
k = 5
 
# function call.
reverseFirstK(q, k)
 
print(*q)

C#

using System;
using System.Collections.Generic;
 
public class ReverseFirstKQueue {
    // Function to reverse first k elements of the queue
    public static void reverseFirstK(Queue<int> q, int k){
        Stack<int> s = new Stack<int>();
 
        // Dequeue the first k elements of the queue and
        // push them onto a stack
        for (int i = 0; i < k; i++) {
            s.Push(q.Dequeue());
        }
 
        // Pop the elements from the stack and enqueue them
        // back into the queue
        while (s.Count > 0) {
            q.Enqueue(s.Pop());
        }
 
        // Dequeue the remaining elements from the queue and
        // enqueue them back into the queue
        for (int i = 0; i < q.Count - k; i++) {
            q.Enqueue(q.Dequeue());
        }
    }
 
    // Driver code.
    public static void Main(){
        Queue<int> q = new Queue<int>();
        q.Enqueue(10);
        q.Enqueue(20);
        q.Enqueue(30);
        q.Enqueue(40);
        q.Enqueue(50);
        q.Enqueue(60);
        q.Enqueue(70);
        q.Enqueue(80);
        q.Enqueue(90);
        q.Enqueue(100);
 
        int k = 5;
 
        // function call.
        reverseFirstK(q, k);
 
        while (q.Count > 0) {
            Console.Write(q.Peek() + " ");
            q.Dequeue();
        }
    }
}

Javascript

// Javascript program to reverse first
// k elements of a queue using dequeue.
function reverseFirstK(q, k) {
    const d = [];
 
    // Dequeue the first k elements of the queue and push
    // them onto a deque
    for (let i = 0; i < k; i++) {
        d.unshift(q.shift());
    }
 
    // Pop the elements from the deque and enqueue them back
    // into the queue
    while (d.length !== 0) {
        q.push(d.shift());
    }
 
    // Dequeue the remaining elements from the queue and
    // enqueue them back into the queue
    for (let i = 0; i < q.length - k; i++) {
        q.push(q.shift());
    }
}
 
// Driver code.
const q = [];
q.push(10);
q.push(20);
q.push(30);
q.push(40);
q.push(50);
q.push(60);
q.push(70);
q.push(80);
q.push(90);
q.push(100);
 
const k = 5;
 
// function call.
reverseFirstK(q, k);
 
console.log(q.join(' '));

Output

50 40 30 20 10 60 70 80 90 100

Time Complexity: O(n), The time complexity of this approach is O(n) where n is the total number of elements in the queue.

Auxiliary Space: O(k) , The space complexity of this approach is O(k) because we are using a deque to store the first k elements of the queue.

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