dl.c

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

// node creation
struct Node {
  int data;
  struct Node* next;
  struct Node* prev;
};

// insert node at the front
void insertFront(struct Node** head, int data) {
  // allocate memory for newNode
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

  // assign data to newNode
  newNode->data = data;

  // make newNode as a head
  newNode->next = (*head);

  // assign null to prev
  newNode->prev = NULL;

  // previous of head (now head is the second node) is newNode
  if ((*head) != NULL)
    (*head)->prev = newNode;

  // head points to newNode
  (*head) = newNode;
}

// insert a node after a specific node
void insertAfter(struct Node* prev_node, int data) {
  // check if previous node is null
  if (prev_node == NULL) {
    printf("previous node cannot be null");
    return;
  }

  // allocate memory for newNode
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

  // assign data to newNode
  newNode->data = data;

  // set next of newNode to next of prev node
  newNode->next = prev_node->next;

  // set next of prev node to newNode
  prev_node->next = newNode;

  // set prev of newNode to the previous node
  newNode->prev = prev_node;

  // set prev of newNode's next to newNode
  if (newNode->next != NULL)
    newNode->next->prev = newNode;
}

// insert a newNode at the end of the list
void insertEnd(struct Node** head, int data) {
  // allocate memory for node
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

  // assign data to newNode
  newNode->data = data;

  // assign null to next of newNode
  newNode->next = NULL;

  // store the head node temporarily (for later use)
  struct Node* temp = *head;

  // if the linked list is empty, make the newNode as head node
  if (*head == NULL) {
    newNode->prev = NULL;
    *head = newNode;
    return;
  }

  // if the linked list is not empty, traverse to the end of the linked list
  while (temp->next != NULL)
    temp = temp->next;

  // now, the last node of the linked list is temp

  // assign next of the last node (temp) to newNode
  temp->next = newNode;

  // assign prev of newNode to temp
  newNode->prev = temp;
}

// delete a node from the doubly linked list
void deleteNode(struct Node** head, struct Node* del_node) {
  // if head or del is null, deletion is not possible
  if (*head == NULL || del_node == NULL)
    return;

  // if del_node is the head node, point the head pointer to the next of del_node
  if (*head == del_node)
    *head = del_node->next;

  // if del_node is not at the last node, point the prev of node next to del_node to the previous of del_node
  if (del_node->next != NULL)
    del_node->next->prev = del_node->prev;

  // if del_node is not the first node, point the next of the previous node to the next node of del_node
  if (del_node->prev != NULL)
    del_node->prev->next = del_node->next;

  // free the memory of del_node
  free(del_node);
}

// print the doubly linked list
void displayList(struct Node* node) {
  struct Node* last;

  while (node != NULL) {
    printf("%d->", node->data);
    last = node;
    node = node->next;
  }
  if (node == NULL)
    printf("NULL\n");
}

int main() {
  // initialize an empty node
  struct Node* head = NULL;

  insertEnd(&head, 5);
  insertFront(&head, 1);
  insertFront(&head, 6);
  insertEnd(&head, 9);

  // insert 11 after head
  insertAfter(head, 11);

  // insert 15 after the seond node
  insertAfter(head->next, 15);

  displayList(head);

  // delete the last node
  deleteNode(&head, head->next->next->next->next->next);

  displayList(head);
}