# What are Queues in Data Structure?

A queue is a data structure that stores a collection of elements, with two main operations: enqueue, which adds an element to the back of the queue, and dequeue, which removes the front element from the queue. Queues are also known as FIFO (first-in, first-out) data structures, because the first element added to the queue will be the first one to be removed.

A queue is useful for situations where elements need to be processed in a specific order, such as a print queue or a task queue for a computer program. Queues can be implemented using an array, a linked list, or a dynamic array.

In a Queue, we have two main pointers, front and rear, which are used to point the front and rear element of the queue, respectively. The element added first will be the front element, and the element added last will be the rear element.

## Types of Queues

There are several types of queues, each with its own specific use case and implementation:

**Simple Queue:**A basic implementation of a queue with enqueue and dequeue operations.**Circular Queue:**A queue in which the last position is connected to the first position, allowing for continuous rotation of the queue.**Priority Queue:**A queue in which elements are inserted based on their priority, with the highest priority element being removed first.**Double Ended Queue (Dequeue):**A queue with the ability to add and remove elements from both the front and the back.**Block Queue:**A queue that blocks when a consumer tries to dequeue an element if the queue is empty or when a producer tries to enqueue an element if the queue is full.**Blocking Priority Queue:**A queue that blocks when a consumer tries to dequeue an element if the queue is empty or when a producer tries to enqueue an element if the queue is full, and also elements are inserted based on their priority, with the highest priority element being removed first.**Concurrent Queue:**A queue that is thread-safe, allowing for multiple threads to add and remove elements simultaneously without causing race conditions.**Distributed Queue:**A queue that is distributed across multiple machines, allowing for scalability and fault tolerance.

The appropriate type of queue to use depends on the specific requirements of the application.

## Example of Queues

Here is an example of a simple queue implementation in several different programming languages:

```
#include <iostream>
#include <queue>
using namespace std;
int main() {
queue<int> q;
// Enqueue elements
q.push(1);
q.push(2);
q.push(3);
// Dequeue elements
cout << q.front() << endl; // Output: 1
q.pop();
cout << q.front() << endl; // Output: 2
q.pop();
cout << q.front() << endl; // Output: 3
q.pop();
return 0;
}
```

Please note that the above examples are just a basic representation of how a queue can be implemented in various languages. In real-world scenarios, you may want to implement additional functions like checking if the queue is empty, etc.

## Applications of Queues

Queues are a fundamental data structure with a wide range of applications. Here are a few examples of how queues are used in practice:

**Task scheduling:**Queues are used to schedule and manage tasks in a system, such as CPU scheduling in an operating system, or job scheduling in a distributed system.**Communication systems:**Queues are used to buffer and manage communications in systems such as networks, telecommunication systems, and message queues.**Printing:**Queues are used to manage print jobs in a printer spooler, ensuring that print jobs are executed in the order in which they were received.**BFS algorithms:**Queues are used to implement Breadth-first search (BFS) algorithms, which are used to traverse graphs and trees.**Resource allocation:**Queues are used to manage resources in systems such as banks and supermarkets, ensuring that customers are served in the order in which they arrive.**Event-Driven systems:**Queues are used to handle the events in an event-driven system, the event which is added first is served first.**Real-time systems:**Queues are used to manage real-time data streams and ensure that data is processed in a timely manner.**Online Games:**Queues are used to manage players waiting in a lobby to join a game.

These are just a few examples, but Queues are used in many other applications as well. They can be used in a wide variety of programming projects and are a fundamental concept in computer science and many other fields.

## Time and Space Complexity of Queues

The time complexity of the basic queue operations is as follows:

**enqueue(): O(1)**- constant time. Adding an element to the back of the queue is a constant time operation.**dequeue(): O(1)**- constant time. Removing an element from the front of the queue is a constant time operation.

In terms of space complexity, a queue uses O(n) space to store n elements, where n is the number of elements in the queue.

It's worth noting that the time complexity of queue operations can be affected by the underlying data structure used to implement the queue. For example, a linked list-based queue has O(1) time complexity for both enqueue and dequeue operations, while a dynamic array-based queue has amortized O(1) time complexity for enqueue and O(1) for dequeue operations.

Overall, Queue is a linear data structure, it follows the First-In-First-Out (FIFO) principle. The time complexity of the basic operations of Queue is O(1) for both Enqueue and Dequeue operations. The space complexity of Queue is O(n) where n is the number of elements in the Queue.

## Advantages and Disadvantages of Queue

**Advantages of Queue:**

Queues follow the First-In-First-Out (FIFO) principle, which ensures that elements are processed in the order in which they were received.

Queues are useful for scheduling and managing tasks in a system, ensuring that they are executed in a specific order.

Queues are used to buffer and manage communications in systems such as networks, telecommunication systems, and message queues.

Queues are used to implement Breadth-first search (BFS) algorithms, which are used to traverse graphs and trees.

Queues are used to manage resources in systems such as banks and supermarkets, ensuring that customers are served in the order in which they arrive.

**Disadvantages of Queue:**

Queues can consume a lot of memory space if the number of elements stored in the queue is large.

Queues can be slow if the number of elements stored in the queue is large, as the time complexity of the basic operations is O(n) where n is the number of elements in the queue.

Queues can be less efficient than other data structures such as stacks or hash tables in certain situations, such as when elements need to be accessed randomly.

Queues can be less efficient when a large number of elements are removed from the front of the queue.

Queues are not efficient for inserting elements in the middle or at the front of the queue.

### Summary

In summary, Queues are a powerful data structure that can be used in a wide variety of applications. They are simple to implement and have a constant time complexity for enqueue and dequeue operations. However, they have some disadvantages too, such as they consume a lot of memory space, can be slow when the number of elements stored in the queue is large, not efficient for inserting elements in the middle or at the front of the queue, and can be less efficient than other data structures in certain situations.