6.dobly linked list
#include <stdio.h>
#include <stdlib.h>

// Define node structure for doubly linked list
struct Node {
    int data;
    struct Node *prev;
    struct Node *next;
};

struct Node *head = NULL;

// Function to create a new node
struct Node* createNode(int value) {
    struct Node *newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->prev = NULL;
    newNode->next = NULL;
    return newNode;
}

// Insert node at the beginning
void insertAtBeginning(int value) {
    struct Node *newNode = createNode(value);
    if (head == NULL) {
        head = newNode;
    } else {
        newNode->next = head;
        head->prev = newNode;
        head = newNode;
    }
    printf("%d inserted at beginning.\n", value);
}

// Insert node at the end
void insertAtEnd(int value) {
    struct Node *newNode = createNode(value);
    if (head == NULL) {
        head = newNode;
    } else {
        struct Node *temp = head;
        while (temp->next != NULL)
            temp = temp->next;
        temp->next = newNode;
        newNode->prev = temp;
    }
    printf("%d inserted at end.\n", value);
}

// Delete a node by value
void deleteNode(int value) {
    struct Node *temp = head;

    if (head == NULL) {
        printf("List is empty!\n");
        return;
    }

    // If head node is to be deleted
    if (temp->data == value) {
        head = temp->next;
        if (head != NULL)
            head->prev = NULL;
        free(temp);
        printf("%d deleted from list.\n", value);
        return;
    }

    // Search for the node to be deleted
    while (temp != NULL && temp->data != value)
        temp = temp->next;

    if (temp == NULL) {
        printf("%d not found in list.\n", value);
        return;
    }

    if (temp->next != NULL)
        temp->next->prev = temp->prev;

    if (temp->prev != NULL)
        temp->prev->next = temp->next;

    free(temp);
    printf("%d deleted from list.\n", value);
}

// Display the list from start to end
void displayForward() {
    struct Node *temp = head;
    if (temp == NULL) {
        printf("List is empty!\n");
        return;
    }
    printf("List (Forward): ");
    while (temp != NULL) {
        printf("%d <-> ", temp->data);
        temp = temp->next;
    }
    printf("NULL\n");
}

// Display the list from end to start
void displayBackward() {
    struct Node *temp = head;
    if (temp == NULL) {
        printf("List is empty!\n");
        return;
    }
    // Move to last node
    while (temp->next != NULL)
        temp = temp->next;

    printf("List (Backward): ");
    while (temp != NULL) {
        printf("%d <-> ", temp->data);
        temp = temp->prev;
    }
    printf("NULL\n");
}

int main() {
    int choice, value;

    while (1) {
        printf("\n--- Doubly Linked List Operations ---\n");
        printf("1. Insert at Beginning\n");
        printf("2. Insert at End\n");
        printf("3. Delete a Node\n");
        printf("4. Display Forward\n");
        printf("5. Display Backward\n");
        printf("6. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);

        switch (choice) {
            case 1:
                printf("Enter value to insert: ");
                scanf("%d", &value);
                insertAtBeginning(value);
                break;
            case 2:
                printf("Enter value to insert: ");
                scanf("%d", &value);
                insertAtEnd(value);
                break;
            case 3:
                printf("Enter value to delete: ");
                scanf("%d", &value);
                deleteNode(value);
                break;
            case 4:
                displayForward();
                break;
            case 5:
                displayBackward();
                break;
            case 6:
                exit(0);
            default:
                printf("Invalid choice! Try again.\n");
        }
    }

    return 0;
}