Hello there! Today, we will explore Linked Lists, a core data structure crucial for organized data management and establishing relationships between data.

We will cover essential aspects, including an introduction to Linked Lists, their real-world applications, their implementation in C++, and the different operations you can perform on them.

By the end of this lesson, you will be well-equipped to implement and operate Linked Lists using C++. Let's get started!

A Linked List is a linear data structure similar to arrays. However, unlike arrays, they are not stored in contiguous memory locations. Each element in a Linked List is part of a node. A node comprises data and a pointer to the next node in the sequence. This structure facilitates efficient insertions and deletions.

The head is also an essential concept in Linked Lists. It is the first node in the list and serves as a reference to the entire list. The head is a nullptr if the Linked List is empty.

Singly linked lists come up quite often in coding interviews. Interviewers from tech giants, start-ups, and just about any company testing your coding abilities will pose challenges based on this concept.

Here's another interesting point: While singly linked lists might not be extensively used in real-world applications, they form the foundational knowledge for understanding doubly linked lists, which are indeed quite common. A singly linked list contains nodes with a single pointer pointing to the next node, whereas a doubly linked list has nodes with pointers to both the next and the previous nodes.

To begin implementing Linked Lists, we first need to understand the structure of a node, the building block of a Linked List. In C++, we'll create a class to serve as a blueprint for a node.

A Node class mainly consists of data (the data you want to store) and next (a pointer to the next node). In our case, we'll create a Node class to store integer data, with a constructor to initialize the node.

C++
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1class Node {
2public:
3    int data;      // Node data
4    Node* next;    // Pointer to next node
5
6    Node(int data) : data(data), next(nullptr) {}  // Initialize node with data
7};

In this section, we'll learn how to add a new node at the end of our Linked List. We'll create a LinkedList class and implement an append method within it.

C++
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1class Node {
2public:
3    int data;
4    Node* next;
5
6    Node(int data) : data(data), next(nullptr) {}
7};
8
9class LinkedList {
10private:
11    Node* head;    // Pointer to first node
12
13public:
14    LinkedList() : head(nullptr) {}  // Initialize empty list
15
16    void append(int data) {
17        Node* node = new Node(data);  // Create new node
18        if (!head) {
19            head = node;  // Set head if list is empty
20        } else {
21            Node* last = head;
22            while (last->next) {
23                last = last->next;  // Traverse to end
24            }
25            last->next = node;  // Append new node
26        }
27    }
28};

The code begins by creating a new node on the heap, ensuring it persists. The code then checks if head is nullptr, which would be the case for an empty list. If that's true, head is set to the new node, making this new node the first and only node in the list. If the linked list is not empty (head is not nullptr), the code traverses to the end of the list using a while loop and appends the new node after the last node.

Now, what if we want to add a new node at the beginning of our list? We'll write a method addFirst to achieve this operation.

C++
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1class LinkedList {
2private:
3    Node* head;
4
5public:
6    LinkedList() : head(nullptr) {}
7
8    void addFirst(int data) {
9        Node* node = new Node(data); // Create new node
10
11        if (head != nullptr) {
12            node->next = head; // Link new node to the current head
13        }
14        head = node; // Update head to new node
15    }
16};

It simply reassigns the head to the new node, making it the first node in the list.

Removing a node from a Linked List is also an essential functionality. We will add a deleteNode method to our LinkedList class to remove a node with a particular data value.

C++
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1class LinkedList {
2private:
3    Node* head;
4
5public:
6    LinkedList() : head(nullptr) {}
7
8    void deleteNode(int data) {
9        if (head == nullptr) return;  // Check if list is empty
10
11        // Check if the head node is the one to be deleted
12        if (head->data == data) {
13            Node* temp = head;
14            head = head->next;  // Move head to the next node
15            delete temp;  // Free memory of the old head
16            return;
17        }
18
19        Node* current = head;
20        // Traverse the list to find the node to delete
21        while (current->next != nullptr) {
22            if (current->next->data == data) {
23                Node* temp = current->next;
24                current->next = current->next->next;  // Bypass the node to delete
25                delete temp;  // Free memory of the bypassed node
26                return;
27            }
28            current = current->next;  // Move to the next node
29        }
30    }
31};

Deleting a node is straightforward, involving some pointer adjustments. However, locating the node can be time-consuming due to the need to traverse the list. Similar to the append method, we traverse the list to find a node with specific data. Once located, the target node is removed by updating the pointer of the previous node to bypass the target, followed by deallocating the target node's memory.

C++
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1class LinkedList {
2private:
3    Node* head;
4
5public:
6    LinkedList() : head(nullptr) {}
7
8    ~LinkedList() {
9        Node* current = head;  // Start with the head node
10        while (current != nullptr) {  // Iterate until the end of the list
11            Node* nextNode = current->next;  // Store the next node
12            delete current;  // Delete the current node
13            current = nextNode;  // Move to the next node
14        }
15    }
16};

The destructor in the LinkedList class is responsible for cleaning up any dynamically allocated memory to prevent memory leaks. It iterates through the linked list, deleting each Node to free up the memory that was allocated for storing the list's elements. This is crucial because it ensures that when a linked list object is destroyed, all resources it used are properly released, maintaining the efficiency and stability of your program.

While understanding the implementation of Linked Lists is great, it's equally crucial to comprehend the performance of our operations. This understanding generally comes through complexity analysis, examining the time (number of operations) and space (memory used) needed for an operation.

Here's a summary of the performance of particular operations in Linked Lists:

• Accessing an element: It has O(n) time complexity because, in the worst-case scenario, we'd have to traverse through the entire list. The space complexity is O(1) as accessing an element requires only a single pointer for traversal.

• Inserting or deleting a node: It's O(1) if we're adding or removing from the front of the list. However, if it's at the end or in the middle, it would be O(n) because we'd have to traverse the list to find the position. The space complexity of this operation is O(1) since the operations rely on a constant amount of additional space for pointers, regardless of the list size.

• The space complexity of the Linked List is O(n).

The LinkedList class acts as a convenient wrapper to manage operations on a Linked List, making it easy to use and organize. However, it's important to note that the essential part of a Linked List is the Node class and the functions that handle link management. Each node inherently holds a pointer to the next node, which means that Node instances, along with a reference to the head, are all that's needed to build and manage a Linked List with full functionality.

Great job sticking with it throughout this intriguing journey, from understanding the concept of Linked Lists to implementing them in C++, exploring critical operations, and understanding their performance!

Up ahead, we have lined up some practice sessions. These sessions will provide you with real-time experience implementing and manipulating Linked Lists using C++.