Queue Data Structure

A queue is a linear data structure that follows the FIFO (First In, First Out) principle, where the element that is inserted first will be the one to be removed first. This makes queues similar to real-world queues like waiting in line at a ticket counter or a restaurant.

A queue is an Abstract Data Type (ADT) similar to stack, the thing that makes queue different from stack is that a queue is open at both its ends. The data is inserted into the queue through one end and deleted from it using the other end.

Queue

Applications of Queues in Real-World Scenarios

Queues are widely used in various real-world scenarios due to their FIFO nature and efficiency in managing data access:

1. Job Scheduling: Operating systems often use queues to manage processes in a round-robin manner or based on priority levels. Jobs are added to a queue and processed according to their arrival time or priority.
2. Breadth-First Search (BFS): Queues are essential in graph traversal algorithms like BFS, where nodes are processed layer by layer. In BFS, the nodes are explored in the order of their depth levels, using a queue to keep track of the current frontier of exploration.
3. Printer Queue: Print jobs sent to a printer are typically managed using a queue. The jobs are printed in the order they are received, ensuring fairness (FIFO) in processing print requests.
4. Call Center Systems: In call centers, customer service requests are often managed using queues. Calls are answered in the order they are received, ensuring that customers are served fairly.

Basic Operations of Queue

• Enqueue: Add an element to the end of the queue
• Dequeue: Remove an element from the front of the queue
• IsEmpty: Check if the queue is empty
• IsFull: Check if the queue is full
• Peek: Get the value of the front of the queue without removing it

Types of Queue

1. Linear Queue:

• Follows FIFO (First In, First Out) principle.
• Operations: enqueue() to add elements to the rear and dequeue() to remove elements from the front.
• Implemented using arrays or linked lists.
• Simple and straightforward structure.

2. Circular Queue:

• Overcomes the limitation of linear queues where space at the beginning can’t be reused.
• Here the last member is linked to the first, forming a circle-like structure.
• Uses a circular array where the rear and front pointers wrap around.
• Efficient use of space and avoids wastage.
• Operations include enqueue() and dequeue() with special handling of pointers.

3. Priority Queue:

• Each element has a priority associated with it.
• Elements are dequeued based on their priority, not the order of insertion.
• Implemented using binary heaps, arrays, linked lists, or other priority-based data structures.
• Useful in scenarios requiring processing based on urgency or importance.

4. Double-Ended Queue (Deque):

• Supports insertion and deletion from both ends of the queue.
• Can function as a queue (FIFO) or a stack (LIFO) depending on the operation.
• Provides more flexibility than standard queues for certain applications.
• Operations: addFront(), removeFront(), addRear(), removeRear().

5. Deque with Priority:

• Combination of a deque and priority queue.
• Each element has a priority associated with its position in the deque.
• Operations are performed considering both position and priority.
• Useful for managing tasks or events with varying priorities.

6. Bounded Queue:

• Has a fixed maximum capacity.
• Once capacity is reached, further enqueue operations are blocked until space is available (or elements are dequeued).
• Ensures memory management and prevents overflow.
• Commonly used in resource-constrained environments or for controlled data buffering.

7. Blocking Queue:

• Supports blocking operations when attempting to dequeue from an empty queue or enqueue to a full queue.
• Ensures synchronization in concurrent programming.
• Useful in multithreaded environments to manage data flow between producer and consumer threads.
• Operations may block until conditions (like availability of space or elements) are met.

8. Priority Blocking Queue:

• Combines the features of a priority queue and a blocking queue.
• Elements are enqueued with priorities and dequeued according to their priority levels.
• Provides synchronization and priority-based processing capabilities.
• Ensures efficient handling of tasks or events with varying levels of importance or urgency in concurrent environments.

Time Complexity of Queue Operation

Time complexity of various queue operations