Implementation of Queue Using Linked List: Program & its Applications

A linked list is ideal for implementing the queue due to its dynamic size and efficient operations. The time complexity is O(1) for both enqueue (insertion of an element from the back) and dequeue (removal of an element from the front). The linked list doesn’t require any resizing as a benefit to avoid space and memory issues allocated for each element individually. It also helps in shifting elements such as array-based queues during dequeue operations. 

The representation of queue using a linked list:

An ordered linked list consists of nodes that contain:

• Data: The actual data or value.
• Next pointer: A reference to the next node in the list.

For the queue, there are two pointers:

• Front: The front pointer points to the first element in a queue. ‍
• Rear: The rear pointer points to the last element in the queue.

The main operations for a queue using a linked list are:

• Enqueue: Elements can be added to the rear of the queue.
• Dequeue: Elements have to be removed from the front of the queue.
• Peek: The front element of the queue is validated without removing it.‍
• isEmpty: Checking if the queue is empty.

Here is the implementation of the queue using a linked list with all the operations:

#include<iostream>
using namespace std;

struct Node {
    int data;
    Node* next;
};

class Queue {
private:
    Node* front;
    Node* rear;

public:
    Queue() {
        front = rear = nullptr;
    }

    // Enqueue operation
    void enqueue(int value) {
        Node* newNode = new Node();
        newNode->data = value;
        newNode->next = nullptr;
        
        if (rear == nullptr) {
            front = rear = newNode;
        } else {
            rear->next = newNode;
            rear = newNode;
        }
    }

    // Dequeue operation
    int dequeue() {
        if (front == nullptr) {
            cout << "Queue is empty!" << endl;
            return -1;
        }
        Node* temp = front;
        int value = front->data;
        front = front->next;
        
        if (front == nullptr) {
            rear = nullptr;
        }

        delete temp;
        return value;
    }

    // Peek operation
    int peek() {
        if (front == nullptr) {
            cout << "Queue is empty!" << endl;
            return -1;
        }
        return front->data;
    }

    // Check if queue is empty
    bool isEmpty() {
        return front == nullptr;
    }
};

int main() {
    Queue q;
    q.enqueue(2);
    q.enqueue(5);
    q.enqueue(10);
    cout << "Front element: " << q.peek() << endl;
    cout << "Dequeued element: " << q.dequeue() << endl;
    cout << "Front element: " << q.peek() << endl;
    return 0;
}

Output

Front element: 2
Dequeued element: 2
Front element: 5

Here are some of the applications of queue using a linked list:

1. Job Scheduling

Operating Systems use queues for job scheduling where processes are executed in the order they arrive. In this case, the queue helps maintain the order of tasks that need to be executed.

For example, jobs in a printer queue or tasks in a process scheduler in an OS are managed using a queue.

2. Breadth-First Search (BFS)

BFS is a graph traversal algorithm used in many applications like social networks, solving mazes, and routing in networks. A queue is essential in BFS for managing the nodes to be explored next. Using a linked list-based queue ensures that memory is used dynamically as the graph or maze grows.

3. Handling Requests in Web Servers

In web servers, incoming requests are handled in the order they are received. A linked list-based queue can manage the requests, ensuring each one is processed in the order it was received.

For example, a web server processes requests to serve files or respond to API calls.

4. Traffic Management

In a traffic control system, queues can be used to manage cars waiting at traffic lights or intersections. A linked list-based queue can dynamically manage the flow of cars based on real-time traffic data.

In conclusion, a queue implemented using a linked list is a versatile and efficient data structure for a wide range of applications, particularly in systems requiring dynamic memory allocation or real-time task handling. It provides efficient O(1) operations for both enqueue and dequeue, making it an essential tool in many real-world scenarios.

1. What happens when the queue is empty and you try to dequeue?

When you try to dequeue from an empty queue, typically, an error or exception is thrown (e.g., "Queue Underflow"). It's important to handle this condition in your implementation by checking if the queue is empty before performing the dequeue operation.

2. Can a queue implemented with a linked list be used in a multi-threaded environment?

Yes, a queue implemented with a linked list can be used in multi-threaded environments, but you would need to implement synchronization mechanisms (like mutexes or locks) to prevent race conditions and ensure thread safety while performing enqueue and dequeue operations.

3. What is the difference between a queue and a stack in terms of a linked list implementation?

The major difference between a queue and a stack is the order in which elements are removed:

• Queue: FIFO (First In, First Out) which is removed from the front, and added to the rear.
• Stack: LIFO (Last In, First Out) which is removed from the top, and added to the top.