Lecture 217:- Let's Code Reverse for Doubly Linked List

Below is the C++ implementation of the DoublyLinkedList class with a method to reverse 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 reverse() {        if (!head || !head->next) {            return; // Empty list or only one node, no need to reverse        }        Node* current = head;        Node* prevNode = nullptr;        while (current) {            // Swap the "next" and "prev" pointers for the current node            Node* nextNode = current->next;            current->next = prevNode;            current->prev = nextNode;            // Move to the next node in the original direction            prevNode = current;            current = nextNode;        }        // Swap the head and tail pointers after reversing        tail = head;        head = prevNode;    }    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.insertAtHead(3);    list.insertAtHead(4);    list.displayForward(); // Output: 4 -> 3 -> 2 -> 1 -> NULL    list.reverse();    list.displayForward(); // Output: 1 -> 2 -> 3 -> 4 -> NULL    return 0; }

In this implementation, the reverse method is added to the DoublyLinkedList class. This method uses two pointers, current and prevNode, to reverse the "next" and "prev" pointers of each node iteratively. Once the reversing is complete, the head and tail pointers are updated to reflect the new positions.

In the main function, we demonstrate how to use the DoublyLinkedList class by creating a doubly linked list, inserting elements at the head, and then reversing the list using the reverse method. The output will be 4 -> 3 -> 2 -> 1 -> NULL for the initial list and 1 -> 2 -> 3 -> 4 -> NULL after the reversal.