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Last Updated on July 31, 2023 by Mayank Dham

In the vast landscape of data structures, linked lists stand as one of the fundamental and versatile building blocks. Linked list types are one of the most interesting topic as there are four different types of linked list in data structures which have different functionality based on their implementation. They provide an elegant solution to manage and organize data, offering efficiency and flexibility in a wide range of applications. By creating a chain of interconnected nodes, each holding a piece of information, linked lists open up a realm of possibilities for data manipulation and storage.

This article delves into the world of linked lists, types of linked list in data structures, characteristics, and use cases. Whether you are an aspiring computer science student, a seasoned developer, or simply curious about the inner workings of data structures, this comprehensive guide will equip you with a deeper understanding of linked lists and their practical applications. Let’s discuss “what is a linked list first?”.

## What is a Linked List?

A linked list is a linear Data Structure, consisting of a group of nodes stored at random addresses. In a linked list the elements are linked using pointers.
Every node stores the data and address of the next node. Every node consists of 2 parts:

Data: The Data which is stored at a particular address.

Linked list can be represented as the connection of nodes in which each node points to the next node of the list. Below is the representation of the linked list

Till now, we have discussed the array data structure to organize the group of elements that are to be stored individually in the memory. However, Array has several advantages and disadvantages that must be known to decide the data structure that will be used throughout the program.

Linked lists come in various forms, each tailored to suit specific use cases and optimize different operations. Here are the most common types of linked lists:

There are three common types of linked list .

A singly list is a linked list that is unidirectional, i.e. it can be traversed in only one direction starting from the head of the linked list to the end node (tail). In a singly linked list, each node contains data and a reference (usually a pointer) to the next node in the sequence. The last node points to NULL, signifying the end of the list. This type of linked list is simple and efficient for traversing forward but requires sequential access for backward traversal.

A data field.
An address field points to the next node.

C Code: https://ideone.com/ge4Qqi
C++ Code: https://ideone.com/WCuz7J
Java Code: https://ideone.com/EXSH6I
Python Code: https://ideone.com/tz1ebr

In a doubly linked list, each node contains data and two references (pointers) – one to the next node and another to the previous node. This bidirectional linkage enables efficient traversal in both directions, making operations like deletion and insertion at any position more straightforward compared to singly linked lists. However, this comes at the cost of increased memory usage due to the additional pointers.

A data field.
Two address fields next and prev, next points to the immediate next node in the linked list, and prev points to the immediate previous node in the linked list.

C Code: https://ideone.com/xG9p35
C++ Code: https://ideone.com/wHDybr
Java Code: https://ideone.com/b2x6gd
Python Code: https://ideone.com/HxJpDQ

In a Circular Linked List, instead of the last node pointing to NULL, the last node points to the head of the linked list. Hence, the name circular linked list. The main advantage of a circular linked list is that we can consider any node as the starting node and traverse the list. This circular structure allows seamless rotation through the elements, which is beneficial in applications like scheduling tasks or implementing circular buffers.

C Code: https://ideone.com/Ob27Hf
C++ Code: https://ideone.com/CdNuSl
Java Code: https://ideone.com/FPJfA9
Python Code: https://ideone.com/aN9gOY

Now, you might have the question, why should we use linked lists over arrays?

Although using arrays, we can store the same types of data; we can access elements directly using the index; however, they have the following drawbacks.

1. The arrays have a fixed size: As a result, we must know the maximum amount of elements ahead of time. In addition, regardless of use, the allocated memory is always equal to the maximum limit.
2. Inserting a new element into an array at some middle position is costly since space must be made for the new elements, and old elements must be shifted to make room.
3. Also deleting an element from an array is costly as it too requires shifting of array elements.

## Basic Operations of Linked List:

Insertion: This operation is used to add an element to the linked list.
Insertion of a node of a linked list can be on three positions i.e. Insertion at the beginning, Insertion at the end, and Insertion in the middle of the list.

Deletion: Deletion operations are used to remove an element from the beginning of the linked list. You can also do delextion in the linked list in three ways either from the end, beginning, or from a specific position.

Search: A search operation is used to search an element using the given key.The search operation is done to find a particular element in the linked list. If the element is found in any location, then it returns. Else, it will return null.

Display: Display operation is used to display the linked list.display() will display the nodes present in the list.

Now, let’s see the time and space complexity of the linked list for the operations search, insert, and delete.

### Time Complexity

Operations Average case time complexity Worst-case time complexity
Insertion O(1) O(1)
Deletion O(1) O(1)
Search O(n) O(n)

Where ‘n’ is the number of nodes in the given tree.

### Space Complexity

Operations Space complexity
Insertion O(n)
Deletion O(n)
Search O(n)

The space complexity of the linked list is O(n).

• Dynamic Data Structure: Linked List being a dynamic data structure can shrink and grow at the runtime by deallocating or allocating memory. So, there is no need for initial size.
• No Memory Wastage: As the size of a linked list can grow or shrink at runtime, there is no memory wastage. Only the required memory is allocated.
• Implementation: Some very helpful data structures like queues and stacks can easily be implemented using a Linked List.
• Insertion and Deletion Operation: In a Linked List, insertion and deletion operations are quite easy, as there is no need to shift every element after insertion or deletion. Only the address present in the pointers needs to be updated.

• Memory Usage: The memory required by a linked list is more than the memory required by an array, as there is also a pointer filed along with the data field in the linked list. The pointer field requires memory to store the address of the next node.
• Random Access: To access nodes at index x in a linked list, we have to traverse through all the nodes before it. In the case of an array, we can directly access an element at index x using arr[x].
• Reverse Traversal: In a singly linked list, reverse traversal is not possible, as every node stores only the address of the next node. But in the case of a doubly-linked list, reverse traversal is possible, but it consumes more memory, as we have to allocate extra memory to store the previous pointer.

## Applications of the linked list:

• Dynamic Data Structures: Linked lists are the foundation of many dynamic data structures, such as stacks, queues, deques, and hash tables. Their ability to efficiently handle insertions and deletions at arbitrary positions makes them ideal for implementing these data structures.
• Memory Management: Linked lists are commonly used in memory management systems to maintain a list of free memory blocks and allocated memory chunks. When memory is allocated or deallocated, linked lists are updated to reflect the changes.
• File Management Systems: Linked lists are useful in file systems for managing directories and files. Each node in the linked list represents a file or directory, and the links connect them in a hierarchical manner.
• Task Scheduling: Circular linked lists are employed in task scheduling algorithms, where processes or tasks are organized in a circular manner, allowing for efficient time-slicing and execution.
• Graph Representation: Linked lists can be used to represent graphs, where each node in the linked list corresponds to a vertex, and the list contains the vertices connected to that vertex.
• Polynomial Manipulation: Linked lists are used to store and manipulate polynomials, where each node represents a term in the polynomial.
• Undo/Redo Functionality: Linked lists are useful for implementing undo and redo functionality in applications where users can perform actions and then revert or redo those actions in a sequential manner.
• Music and Playlist Management: Music playlist applications use linked lists to create playlists and enable smooth navigation between songs.
• Navigation Systems: Linked lists are used in GPS navigation systems to store a sequence of waypoints or directions to guide users along a route.
• Symbol Table in Compilers: Linked lists are used to implement symbol tables in compilers, where each node represents a symbol, and the linked list helps in efficient searching and management of symbols during the compilation process.

Conclusion
In conclusion, linked lists are a foundational data structure that plays a crucial role in computer science and software development. They offer a flexible and efficient way to organize and manage data, making them valuable tools in various applications. Throughout this article, we explored the different types of linked lists, each with its own unique characteristics and advantages.
Choosing the right type of linked list depends on the specific requirements of your application. Whether you need to optimize for insertion, deletion, searching, or memory usage, understanding the strengths and weaknesses of each type is crucial. By leveraging the power of linked lists effectively, developers can design efficient algorithms, improve data management, and enhance the overall performance of their software systems.

## FAQ (Frequently Asked Questions) on Types of Linked List in Data Structure:

Here are a few FAQs on different types of linked list in data structure.

Q: When should I use a linked list over other data structures?
A: Linked lists are particularly useful when you require dynamic memory allocation and frequent insertions or deletions at arbitrary positions. They shine in scenarios where the size of the data can change frequently and when a flexible data structure is needed. For sequential access, arrays might be a better choice due to their cache-friendliness.

A: Circular linked lists offer efficient navigation in both directions, making them ideal for situations that require cyclical traversal, like implementing circular buffers or managing cyclic tasks. Additionally, they simplify operations at both ends of the list, enabling easy insertions and deletions.

Q: Can linked lists be used for searching and sorting efficiently?
A: While linked lists excel in insertions and deletions, they are not the best choice for searching and sorting in most cases. Arrays or other data structures, such as binary search trees or hash tables, are typically more efficient for these operations.

Q: What is the advantage of using a skip list over a regular linked list for searching?
A: Skip lists offer faster search operations compared to regular linked lists. By creating multiple layers of linked lists with fewer elements at each higher level, skip lists reduce the search space dramatically, leading to improved time complexity for searching.

Q: Can I combine different types of linked lists in a single application?
A: Yes, you can certainly use multiple types of linked lists in a single application based on your specific requirements. For example, you may use a doubly linked list for efficient insertions and deletions and a circular linked list for cyclic navigation.

Q: Are linked lists a memory-efficient data structure?
A: While linked lists offer dynamic memory allocation, each node incurs additional overhead due to its pointers. This can lead to higher memory usage compared to arrays or other data structures. If memory efficiency is a primary concern, you may consider using other compact data structures.