Linked list vs Queue

Posted by Akshay Sharma on February 26th, 2023

Linked lists and queues are two commonly used data structures in computer science and programming. While both can be used to store and process collections of elements, they have different strengths and weaknesses, and are suited for different use cases. In this comparison, we will examine the similarities and differences between linked lists and queues, and discuss when each one is the best choice.

• Linked list

A linked list is a linear data structure that consists of a chain of nodes, where each node contains an element and a reference (or "link") to the next node in the list.

Each node in a linked list is typically represented as an object with two fields: data and next. The data field contains the element stored in the node, and the next field is a reference to the next node in the list. The last node in the list has a next field set to null, indicating the end of the list.

Linked lists can be used to implement various abstract data types, such as stacks, queues, and associative arrays. They can also be used in various algorithms, such as searching and sorting, and in graph algorithms such as depth-first search and breadth-first search.

Below are some advantages of linked list are-

1. Dynamic Size: Linked lists have a dynamic size, meaning that elements can be added or removed from the list at any time, and the size of the list will automatically adjust. This is in contrast to arrays, which have a fixed size that must be specified when they are created.

2. Efficient Insertion and Deletion: Inserting or deleting elements in the middle of a linked list is efficient, as it only requires updating the links between the surrounding nodes. In contrast, inserting or deleting elements in the middle of an array requires shifting all the elements after the insertion or deletion point, which can be time-consuming.

3. Easy to add or remove elements from the middle: Because of the dynamic size and efficient insertion and deletion, linked lists are particularly well-suited for applications that require the frequent addition or removal of elements from the middle of the list.

4. No Wasteful Space Allocation: Linked lists allocate memory only when it is needed, so there is no wastage of memory due to unused elements in an array.

5. Ease of Implementation: Linked lists are relatively easy to implement compared to arrays, especially when it comes to adding or removing elements from the list.

However, linked lists also have some disadvantages, such as the overhead of creating and maintaining the links between nodes, and the difficulty of accessing elements at a specific index. In such cases, arrays may be a better choice.

• Queue 

A queue is a data structure that implements the First-In-First-Out (FIFO) principle, meaning that elements are added to the end of the queue and removed from the front. This makes a queue a useful data structure for representing a collection of elements that have to be processed in order.

A queue can be implemented using an array or a linked list. When implemented as an array, the front and rear indices are used to keep track of the front and rear of the queue, respectively. The front index points to the first element in the queue, and the rear index points to the position where the next element will be added.

When implemented as a linked list, each node in the list represents an element in the queue, and the first element in the queue is the first node in the linked list. To enqueue an element, a new node is created with the element and added to the end of the linked list. To dequeue an element, the first node in the linked list is removed and its data field is returned.

Queues are used in various applications, such as managing incoming requests in a server, scheduling processes in an operating system, and simulating real-life scenarios such as customers waiting in line at a checkout counter. They can also be used in algorithms, such as breadth-first search, to keep track of the elements to be processed.

There are several types of queues, including:

1. Simple Queue: A simple queue is a basic implementation of a queue data structure, where elements are added to the rear and removed from the front in a First-In-First-Out (FIFO) manner.

2. Circular Queue: A circular queue is a type of queue where the last element points back to the first element, forming a circular chain. This allows for efficient use of memory and eliminates the need to shift elements when the rear of the queue reaches the end of the array. There are several  advantages of a circular queue.

3. Priority Queue: A priority queue is a type of queue where elements are processed based on their priority, rather than their order of arrival. In a priority queue, elements with higher priority are dequeued before elements with lower priority.

4. Double-Ended Queue (Deque): A double-ended queue, also known as a deque, is a type of queue where elements can be added or removed from both the front and the rear. This makes it possible to implement both stacks and queues using a single data structure.

5. Blocking Queue: A blocking queue is a type of queue where a thread that tries to dequeue from an empty queue is blocked until an element becomes available. This can be useful in multithreaded programming when it is necessary to coordinate the processing of elements between multiple threads.

6. Concurrent Queue: A concurrent queue is a type of queue that can be safely accessed by multiple threads simultaneously. This is useful in multithreaded programming where multiple threads may need to access the same queue concurrently.

Each type of queue is suited for different use cases and applications, and the choice of which type to use depends on the requirements of the specific scenario.

In conclusion, linked lists and queues are two useful data structures that can be used to store and process collections of elements. While they have some similarities, such as the ability to store elements in a specific order, they are also different in many ways. Linked lists are best for scenarios where efficient insertion and deletion of elements is required, while queues are best for scenarios where elements need to be processed in a specific order, such as First-In-First-Out (FIFO). Ultimately, the choice between linked lists and queues depends on the specific requirements of the application and the trade-offs between time and space complexity.