Lecture 213:- Let's Code Insert for Doubly Linked List

Below is the C++ implementation of the DoublyLinkedList class with methods to insert elements at the head, tail, and at a specific position in the doubly linked list:

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#include <iostream> class Node { public:    int data;    Node* next;    Node* prev;    Node(int data) : data(data), next(nullptr), prev(nullptr) {} }; class DoublyLinkedList { private:    Node* head;    Node* tail; public:    DoublyLinkedList() : head(nullptr), tail(nullptr) {}    ~DoublyLinkedList() {        Node* current = head;        while (current) {            Node* temp = current;            current = current->next;            delete temp;        }    }    void insertAtHead(int data) {        Node* newNode = new Node(data);        if (!head) {            head = newNode;            tail = newNode;        } else {            newNode->next = head;            head->prev = newNode;            head = newNode;        }    }    void insertAtTail(int data) {        Node* newNode = new Node(data);        if (!tail) {            head = newNode;            tail = newNode;        } else {            newNode->prev = tail;            tail->next = newNode;            tail = newNode;        }    }    void insertAtPosition(int data, int position) {        if (position <= 0) {            insertAtHead(data);        } else if (!head) {            std::cout << "List is empty. Cannot insert at position." << std::endl;        } else {            Node* newNode = new Node(data);            Node* current = head;            int count = 0;            while (count < position - 1 && current->next) {                current = current->next;                count++;            }            if (count == position - 1) {                newNode->prev = current;                newNode->next = current->next;                if (current->next) {                    current->next->prev = newNode;                } else {                    tail = newNode;                }                current->next = newNode;            } else {                std::cout << "Invalid position. Insertion failed." << std::endl;                delete newNode;            }        }    }    void displayForward() {        if (!head) {            std::cout << "List is empty." << std::endl;            return;        }        Node* current = head;        while (current) {            std::cout << current->data << " -> ";            current = current->next;        }        std::cout << "NULL" << std::endl;    }    void displayBackward() {        if (!tail) {            std::cout << "List is empty." << std::endl;            return;        }        Node* current = tail;        while (current) {            std::cout << current->data << " -> ";            current = current->prev;        }        std::cout << "NULL" << std::endl;    } }; int main() {    DoublyLinkedList list;    list.insertAtHead(1);    list.insertAtHead(2);    list.insertAtTail(3);    list.insertAtPosition(4, 1);    list.displayForward(); // Output: 2 -> 4 -> 1 -> 3 -> NULL    list.displayBackward(); // Output: 3 -> 1 -> 4 -> 2 -> NULL    return 0; }

This implementation of the DoublyLinkedList class contains the following methods:

1. insertAtHead: Inserts a new node at the beginning of the doubly linked list.
2. insertAtTail: Inserts a new node at the end of the doubly linked list.
3. insertAtPosition: Inserts a new node at the specified position in the doubly linked list. The position is 0-based, meaning the first element has position 0.

The displayForward and displayBackward methods are used to display the elements of the doubly linked list in the forward and backward directions, respectively.

In the main function, we demonstrate how to use the DoublyLinkedList class by creating a doubly linked list, inserting elements at the head, tail, and at a specific position, and displaying the doubly linked list in both forward and backward directions. The output will be 2 -> 4 -> 1 -> 3 -> NULL for forward direction and 3 -> 1 -> 4 -> 2 -> NULL for backward direction after the insertions.