doubly.c

#include<stdio.h>
#include<stdlib.h>

struct Node {
    int data;
    struct Node *next;
    struct Node *prev; // New pointer for DLL
};

void insertAtStart(int data, struct Node **head) {
    struct Node *newnode = malloc(sizeof(struct Node));
    newnode->data = data;
    newnode->prev = NULL;
    newnode->next = *head;

    if (*head != NULL) {
        (*head)->prev = newnode; // Link old head back to new node
    }
    
    *head = newnode;
}

void insertAtEND(int data, struct Node **head) {
    struct Node *newnode = malloc(sizeof(struct Node));
    newnode->data = data;
    newnode->next = NULL;

    if (*head == NULL) {//incase list is empty it creates the list's first element
        newnode->prev = NULL;
        *head = newnode;
        return;
    }

    struct Node *temp = *head;
    while(temp->next != NULL) {
        temp = temp->next;
    }

    temp->next = newnode;
    newnode->prev = temp; // Link new node back to old last node
}

void insertAtPosition(int data, struct Node **head, int index) {
    if (index == 0) {
        insertAtStart(data, head);
        return;
    }

    struct Node *temp = *head;
    int counter = 0;

    while(temp != NULL && counter < index - 1) {
        temp = temp->next;
        counter++;
    }

    if (temp == NULL) return; // Index out of range

    struct Node *newnode = malloc(sizeof(struct Node));
    newnode->data = data;

    newnode->next = temp->next;
    newnode->prev = temp;

    if (temp->next != NULL) {
        temp->next->prev = newnode; // Link succeeding node back to new node
    }
    temp->next = newnode;
}

void deleteAtFront(struct Node **head) {
    if (*head == NULL) return;

    struct Node *old = *head;
    *head = (*head)->next;

    if (*head != NULL) {
        (*head)->prev = NULL; // New head has no previous
    }

    free(old);
}

void deleteAtEnd(struct Node **head) {
    if (*head == NULL) return;

    if ((*head)->next == NULL) {
        free(*head);
        *head = NULL;
        return;
    }

    struct Node *temp = *head;
    while(temp->next != NULL) {
        temp = temp->next;
    }

    temp->prev->next = NULL; // The second-to-last node now points to NULL
    free(temp);
}

void deleteAtIndex(struct Node **head, int index) {
    if (*head == NULL) return;

    if (index == 0) {
        deleteAtFront(head);
        return;
    }

    struct Node *temp = *head;
    int counter = 0;

    while(temp != NULL && counter < index) {
        temp = temp->next;
        counter++;
    }

    if (temp == NULL) return; // Index out of range

    // Unlinking the node
    temp->prev->next = temp->next;
    if (temp->next != NULL) {
        temp->next->prev = temp->prev;
    }

    free(temp);
}

void display(struct Node *head) {
    struct Node *temp = head;
    printf("Forward:  ");
    while(temp != NULL) {
        printf("%d -> ", temp->data);
        if (temp->next == NULL) break;
        temp = temp->next;
    }
    printf("NULL\n");

    // Proving it's a DLL by traversing backward
    printf("Backward: ");
    while(temp != NULL) {
        printf("%d -> ", temp->data);
        temp = temp->prev;
    }
    printf("NULL\n");
}

int main() {
    struct Node *head = NULL;

    insertAtEND(20, &head);
    insertAtEND(30, &head);
    insertAtEND(40, &head);
    insertAtStart(10, &head);
    insertAtPosition(25, &head, 2);

    display(head);

    deleteAtIndex(&head, 2); // Removes 25
    printf("\nAfter deleting index 2:\n");
    display(head);

    return 0;
}