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

// Define the DLL Node
struct Node {
  int data;
  struct Node *prev; // Pointer to previous node
  struct Node *next; // Pointer to next node
};

// Global Head Pointer
struct Node *head = NULL;

// a) Insert a node (Create/Append) - 15 Marks
// We insert at the END to simulate "Creating" a list naturally
void insert(int value) {
  struct Node *temp;

  // 1. Allocate memory
  struct Node *newNode = (struct Node *)malloc(sizeof(struct Node));
  if (newNode == NULL) {
    printf("Memory allocation failed\n");
    return;
  }

  // 2. Set data
  newNode->data = value;
  newNode->next = NULL; // Since it will be the last node

  // 3. Case 1: List is Empty
  if (head == NULL) {
    newNode->prev = NULL; // First node has no prev
    head = newNode;
    printf("Node %d inserted as head.\n", value);
    return;
  }

  // 4. Case 2: List has data. Traverse to the end.
  temp = head;
  while (temp->next != NULL) {
    temp = temp->next;
  }

  // 5. Link the new node
  temp->next = newNode; // Old last node points to new node
  newNode->prev = temp; // New node points back to old last node

  printf("Node %d inserted at end.\n", value);
}

// b) Delete a node (by value) - 15 Marks
void delete(int value) {
  struct Node *temp = head;

  // 1. Check if list is empty
  if (head == NULL) {
    printf("List is empty.\n");
    return;
  }

  // 2. Search for the node
  while (temp != NULL && temp->data != value) {
    temp = temp->next;
  }

  // 3. If node not found
  if (temp == NULL) {
    printf("Element %d not found.\n", value);
    return;
  }

  // --- Deletion Logic ---

  // Case A: Deleting the Head Node
  if (temp == head) {
    head = head->next; // Move head forward
    if (head != NULL) {
      head->prev = NULL; // New head's prev must be NULL
    }
  }
  // Case B: Deleting a Middle or Last Node
  else {
    // Link the previous node to the next node
    temp->prev->next = temp->next;

    // If there IS a next node (i.e., we are not deleting the last node),
    // Link the next node back to the previous node
    if (temp->next != NULL) {
      temp->next->prev = temp->prev;
    }
  }

  // 4. Free memory
  free(temp);
  printf("Node %d deleted.\n", value);
}

// c) Display the list - 5 Marks
void display() {
  struct Node *temp = head;

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

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

int main() {
  int choice, val;
  printf("--- Doubly Linked List Program ---\n");

  while (1) {
    printf("\n1. Insert (Create)\n2. Delete\n3. Display\n4. Exit\n");
    printf("Enter choice: ");
    scanf("%d", &choice);

    switch (choice) {
    case 1:
      printf("Enter value: ");
      scanf("%d", &val);
      insert(val);
      break;
    case 2:
      printf("Enter value to delete: ");
      scanf("%d", &val);
      delete(val);
      break;
    case 3:
      display();
      break;
    case 4:
      return 0;
    default:
      printf("Invalid Choice\n");
    }
  }
  return 0;
}