Question

Here is a program it is working

#include <stdio.h>
#include <stdlib.h>
struct node {
    int data;
    struct node *next, *prev;
};
struct node *root = NULL;
void push(int);
void pop(void);
struct node *create_node(int);
void travel(void);
int main()
{
    int i, j, choice, count;
    printf("enter choice\n");
    scanf("%d", &choice);
    count = 0;
    while (choice == 1) {
        printf("enter a data element");
        scanf("%d", &j);
        if (count == 0) {
            root = (struct node *)malloc(sizeof(struct node));
            root->next = NULL;
            root->data = j;
        } else
            push(j);
        count++;
        printf("enter choice\n");
        scanf("%d", &choice);
    }
    printf("the link list is \n");
//travel function to be created
    travel();
}

void push(int data)
{
    struct node *t1;
    t1 = root;
    while (t1->next != NULL) {
        t1 = t1->next;
    }
    t1->next = create_node(data);
}

void pop()
{
}

void travel(void)
{
    struct node *t1;
    t1 = root;
    while (t1->next != NULL) {
        printf("%d ", t1->data);
        t1 = t1->next;
    }
    printf("%d ", t1->data);
}

struct node *create_node(int data)
{
    struct node *p = (struct node *)malloc(sizeof(struct node));
    p->data = data;
    p->next = NULL;
    p->prev = NULL;
    return p;
}

the above program is fully working,I have used a global pointer root.
My problem is if I do not want to use a global pointer root here then how do I maintain
that list because each time I will have to return the root of list in my push pop functions
is there any other way to achieve the same?

Answer 1

The simplest way to achieve this is to pass a pointer to the root node pointer to each of your functions:

void push(struct node **root, int data) { ... }
void pop(struct node **root) { ... }
void travel(struct node *root) { ... }

So, in your main function you might declare a local variable to hold the root pointer:

struct node *root = NULL;

and then when you call push, for example, you pass the address of the root poiner:

push(&root, data);

I strongly recommend that you fix your push and travel functions so that they are robust to the root pointer being NULL. This was discussed in a previous question of yours and you should heed the advice.

If you did that then you could get rid of the test for count being zero and the associated special case code. You would then replace this:

if (count == 0) {
    root = (struct node *)malloc(sizeof(struct node));
    root->next = NULL;
    root->data = j;
} else
    push(&root, j);

with this:

push(&root, j);

To drive home the message, your new push would look like this:

void push(struct node **root, int data)
{
    if (*root == NULL)
        *root = create_node(data);
    else
    {
        struct node *last = *root;
        while (last->next != NULL) {
            last = last->next;
        }
        last->next = create_node(data);
    }
}

You would need to modify travel also to include a check for the root node being NULL. I will leave that as an exercise for you.

Maintaining both head and tail pointers could be a better approach since it would avoid so many list traversals.