Doubly Linked List Introduction and Insertion

// 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;

}

 

 Output:

 Created DLL is:
Traversal in forward direction
 1  7  8  6  4
Traversal in reverse direction
 4  6  8  7  1