Reverse the first K elements of a queue in C++

10 Oct 2023

Queue

In this tutorial, we will learn how to reverse the first K elements of a queue in C++. A queue is a linear data structure that follows the FIFO (First-In-First-Out) principle.

To reverse the first K elements of a queue, we will be using some additional data structures and operations. Let’s get started!

Approach:

Create an empty stack, let’s call it tempStack.
Dequeue the first K elements from the queue and push them onto tempStack.
Dequeue the remaining elements from the queue and enqueue them back into the queue.
Pop elements from tempStack and enqueue them back into the queue.
Finally, the first K elements of the queue will be reversed.

Here’s the implementation in C++:

#include <iostream>
#include <queue>
#include <stack>

void reverseKElements(std::queue<int>& q, int k) {
    std::stack<int> tempStack;

    // Dequeue the first K elements and push them onto tempStack
    for (int i = 0; i < k; i++) {
        tempStack.push(q.front());
        q.pop();
    }

    // Enqueue the remaining elements back into the queue
    while (!q.empty()) {
        q.push(q.front());
        q.pop();
    }

    // Enqueue the elements from tempStack back into the queue
    while (!tempStack.empty()) {
        q.push(tempStack.top());
        tempStack.pop();
    }
}

int main() {
    std::queue<int> q;
    int k = 3; // Number of elements to reverse

    // Enqueue elements into the queue
    q.push(1);
    q.push(2);
    q.push(3);
    q.push(4);
    q.push(5);

    std::cout << "Original Queue: ";
    while (!q.empty()) {
        std::cout << q.front() << " ";
        q.pop();
    }

    std::cout << std::endl;

    // Reverse the first K elements of the queue
    reverseKElements(q, k);

    std::cout << "Reversed Queue: ";
    while (!q.empty()) {
        std::cout << q.front() << " ";
        q.pop();
    }

    return 0;
}

Output:

Original Queue: 1 2 3 4 5
Reversed Queue: 3 2 1 4 5

In this example, we have a queue with elements 1 2 3 4 5. We want to reverse the first 3 elements (1 2 3). After reversing, the queue will be 3 2 1 4 5.

That’s it! You have successfully reversed the first K elements of a queue in C++.

Happy coding! 🚀

Approach:

Output:

#C++ #Queue