// A complete working C program to demonstrate all insertion methods 
#include <stdio.h> 
#include <stdlib.h> 
  
// A linked list node 
struct Node { 
    int data; 
    struct Node* next; 
    struct Node* prev; 
}; 
  
/* Given a reference (pointer to pointer) to the head of a list 
   and an int, inserts a new node on the front of the list. */
void push(struct Node** head_ref, int new_data) 
{ 
    /* 1. allocate node */
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); 
  
    /* 2. put in the data  */
    new_node->data = new_data; 
  
    /* 3. Make next of new node as head and previous as NULL */
    new_node->next = (*head_ref); 
    new_node->prev = NULL; 
  
    /* 4. change prev of head node to new node */
    if ((*head_ref) != NULL) 
        (*head_ref)->prev = new_node; 
  
    /* 5. move the head to point to the new node */
    (*head_ref) = new_node; 
} 
  
/* Given a node as prev_node, insert a new node after the given node */
void insertAfter(struct Node* prev_node, int new_data) 
{ 
    /*1. check if the given prev_node is NULL */
    if (prev_node == NULL) { 
        printf("the given previous node cannot be NULL"); 
        return; 
    } 
  
    /* 2. allocate new node */
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); 
  
    /* 3. put in the data  */
    new_node->data = new_data; 
  
    /* 4. Make next of new node as next of prev_node */
    new_node->next = prev_node->next; 
  
    /* 5. Make the next of prev_node as new_node */
    prev_node->next = new_node; 
  
    /* 6. Make prev_node as previous of new_node */
    new_node->prev = prev_node; 
  
    /* 7. Change previous of new_node's next node */
    if (new_node->next != NULL) 
        new_node->next->prev = new_node; 
} 
  
/* Given a reference (pointer to pointer) to the head 
   of a DLL and an int, appends a new node at the end  */
void append(struct Node** head_ref, int new_data) 
{ 
    /* 1. allocate node */
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); 
  
    struct Node* last = *head_ref; /* used in step 5*/
  
    /* 2. put in the data  */
    new_node->data = new_data; 
  
    /* 3. This new node is going to be the last node, so 
          make next of it as NULL*/
    new_node->next = NULL; 
  
    /* 4. If the Linked List is empty, then make the new 
          node as head */
    if (*head_ref == NULL) { 
        new_node->prev = NULL; 
        *head_ref = new_node; 
        return; 
    } 
  
    /* 5. Else traverse till the last node */
    while (last->next != NULL) 
        last = last->next; 
  
    /* 6. Change the next of last node */
    last->next = new_node; 
  
    /* 7. Make last node as previous of new node */
    new_node->prev = last; 
  
    return; 
} 
  
// This function prints contents of linked list starting from the given node 
void printList(struct Node* node) 
{ 
    struct Node* last; 
    printf("\nTraversal in forward direction \n"); 
    while (node != NULL) { 
        printf(" %d ", node->data); 
        last = node; 
        node = node->next; 
    } 
  
    printf("\nTraversal in reverse direction \n"); 
    while (last != NULL) { 
        printf(" %d ", last->data); 
        last = last->prev; 
    } 
} 
  
/* Drier program to test above functions*/
int main() 
{ 
    /* Start with the empty list */
    struct Node* head = NULL; 
  
    // Insert 6.  So linked list becomes 6->NULL 
    append(&head, 6); 
  
    // Insert 7 at the beginning. So linked list becomes 7->6->NULL 
    push(&head, 7); 
  
    // Insert 1 at the beginning. So linked list becomes 1->7->6->NULL 
    push(&head, 1); 
  
    // Insert 4 at the end. So linked list becomes 1->7->6->4->NULL 
    append(&head, 4); 
  
    // Insert 8, after 7. So linked list becomes 1->7->8->6->4->NULL 
    insertAfter(head->next, 8); 
  
    printf("Created DLL is: "); 
    printList(head); 
  
    getchar(); 
    return 0; 
}