Rotating a Linked List C - c

Question

I am currently solving sum problems of list and function and i came across this question i.e rotate a linked list by k anti clockwise.
Here is the code for same
void rotate_k(struct list *node,int k)
{
int count=0;
struct list *knode,*ptr=node;
while(ptr!=NULL && count < k)
{
ptr=ptr->next;
count++;
}
knode=ptr;
while(ptr->next!=NULL)
{
ptr=ptr->next;
}
ptr->next =node;
node=knode->next;
knode->next=NULL;
}
Lets say if input is 1->2->3->4->5->6 and k=4.
The output should be 5->6->1->2->3->4 but the code gives the output 1->2->3->4->5 .
Help needed :)

Answer 1

You are not modifying the original list (node parameter)
struct list *rotate_k(struct list *node,int k)
{
int count=0;
struct list *knode,*ptr=node;
while(ptr!=NULL && count < k)
{
ptr=ptr->next;
count++;
}
knode=ptr;
while(ptr->next!=NULL)
{
ptr=ptr->next;
}
ptr->next =node;
node=knode->next;
knode->next=NULL;
return knode; //<-- THIS IS THE NEW LIST
}
Also, knode->next=NULL is strange; you should do at the node that is (was) previous to knode (this is what is deleting the 6 from your results).

Answer 2

SJuan's method is correct but if you want to do it your way without using a return value, then you need to use a double pointer for node. Remember, C makes copies of variables you pass into a function. If the original root node was a pointer (which I'm assuming it was) than you need to make a pointer to a pointer otherwise you are just making changes to a copy of the root node pointer, not the actual root node pointer.
void rotate_k(struct list **node, int k)
{
int count = 0;
struct list *knode, *ptr = *node;
while(ptr != NULL && count < k)
{
ptr = ptr->next;
count++;
}
knode = ptr;
while(ptr->next != NULL)
{
ptr = ptr->next;
}
ptr->next = *node;
*node = knode->next;
knode->next = NULL;
}

Answer 3

void rotate_list_right(listnode** head, int k)
{
if( !head || !*head )
{
printf( "\nrotate_list_right: empty list = so return \n" );
return;
}
if( k < 1 )
{
printf( "\nrotate_list_right:invalid input: k must be >= 1 \n" );
return;
}
listnode* post = *head;
listnode* curr = *head;
/* move post by k nodes */
while(k--)
{
post = post->next;
if( !post ) /* k is bigger than length of the list */
{
printf( "\nrotate_list_right:invalid input: k must be smaller than list size \n" );
return;
}
}
/* move curr to kth-last node */
while(post->next)
{
curr = curr->next;
post = post->next;
}
/* currs' next is new header */
listnode* tmp = *head;
*head = curr->next;
curr->next = 0;
//join post
post->next = tmp;
}