Middle node ptr of a linked list

How to find the middle node ptr of a linked list by one traverse through the linked list?

[B]Question asked by visitor seshu babu k[/b]

#include<stdio.h>

typedef struct node
{
int value;
struct node *next;
struct node *prev;
}mynode ;



void add_node(struct node **head, int value);
void print_list(char *listName, struct node *head);
void getTheMiddle(mynode *head);

// The main function..
int main()
{
mynode *head;
head = (struct node *)NULL;

add_node(&head, 1);
add_node(&head, 10);
add_node(&head, 5);
add_node(&head, 70);
add_node(&head, 9);
add_node(&head, -99);
add_node(&head, 0);
add_node(&head, 555);
add_node(&head, 55);

print_list("myList", head);
getTheMiddle(head);

//getch();
return(0);
}




// This function uses one slow and one fast
// pointer to get to the middle of the LL.
//
// The slow pointer is advanced only by one node
// and the fast pointer is advanced by two nodes!

void getTheMiddle(mynode *head)
{
mynode *p = head;
mynode *q = head;

if(q!=NULL)
{
while((q->next)!=NULL && (q->next->next)!=NULL)
{
p=(p!=(mynode *)NULL?p->next:(mynode *)NULL);
q=(q!=(mynode *)NULL?q->next:(mynode *)NULL);
q=(q!=(mynode *)NULL?q->next:(mynode *)NULL);
}
printf("The middle element is [%d]",p->value);
}
}



// Function to add a node
void add_node(struct node **head, int value)
{
mynode *temp, *cur;
temp = (mynode *)malloc(sizeof(mynode));
temp->next=NULL;
temp->prev=NULL;

if(*head == NULL)
{
*head=temp;
temp->value=value;
}
else
{
for(cur=*head;cur->next!=NULL;cur=cur->next);
cur->next=temp;
temp->prev=cur;
temp->value=value;
}
}

// Function to print the linked list...
void print_list(char *listName, struct node *head)
{
mynode *temp;

printf("\n[%s] -> ", listName);
for(temp=head;temp!=NULL;temp=temp->next)
{
printf("[%d]->",temp->value);
}

printf("NULL\n");

}