Distance between two nodes in circular linked list - c

Question

I want to find the distance between two nodes (number of node between them) in a circular linked list.
Where nodal is :
typedef struct node
{
int data;
struct node *next;
}nodal;
nodal *distance(nodal *start)
{
int n1,n2,d1=0,d2=0,c=0;
if(start==NULL)
{
printf("\nList is empty");
}
else
{
printf("\nEnter the fist node element : ");
scanf("%d",&n1);
printf("\nEnter the second node element : ");
scanf("%d",&n2);
nodal *ptr=start;
while(ptr->next!=start)
{
c++;
if(ptr->data==n1)
{
d1=c;
}
if(ptr->data==n2)
{
d2=c;
}
}
}
printf("The distance between two nodes is %d",(d2-d1));
return start;
}
I doesn't give any output.
Also suppose if circular list contains following data :
1,2,3,4,5,6,7,8,9
And if I give input
First node element as 9
Second node element as 4
Then this algorithm won't work.
Any suggestions for the change ?

Answer 1

Start counting when you find the first node and stop when you find the second one, something like:
int inc = 0, dist = 0;
if (n1 == n2) {
return 0;
}
node = start;
while (node) {
dist += inc;
if ((node->data == n1) || (node->data == n2)) {
if (inc++ == 1) return dist;
}
node = node->next;
if (node == start) break;
}
return 0;

Answer 2

First you have to go to the first element entered, in this process count will not be increased. Then start increasing to count until you reach the second element.
Code is below:
ptr=start;
while(ptr->data!=n1)
{
ptr=ptr->next;
}
//---now we have reached the first number----
while(ptr->data!=n2)
{
count++;
ptr=ptr->next;
}

Answer 3

There are multiple problems in your code:
the test for the end of the list prevents you from testing the last node.
you handle the case where n1 == n2 by returning 0, which may not be what is expected.
the distance between n1 and n2 will be negative if n2 appears before n1, which is incorrect: since the list is circular, if you keep scanning after n1, you will encounter n2 at a positive distance at most after wrapping over the first node.
Here is an alternate solution:
typedef struct node {
int data;
struct node *next;
} nodal;
nodal *distance(nodal *start) {
int n1, n2, d1 = 0, d2 = 0, length = 0;
if (start == NULL) {
printf("List is empty\n");
} else {
printf("\nEnter the fist node element : ");
if (scanf("%d", &n1) != 1) {
printf("invalid input\n");
return start;
}
printf("\nEnter the second node element : ");
if (scanf("%d", &n2) != 1) {
printf("invalid input\n");
return start;
}
nodal *ptr = start;
for (;;) {
length++;
if (ptr->data == n1 && !d1) {
d1 = length;
} else if (ptr->data == n2) {
if (d1) {
d2 = length;
break;
}
if (!d2)
d2 = length;
}
ptr = ptr->next;
if (ptr == start)
break;
}
}
if (d1 && d2) {
int dist = d2 - d1;
if (dist < 0)
dist += length;
printf("The distance between node(%d) and node(%d) is %d\n", d1, d2, dist);
} else {
if (d2)
printf("node(%d) not found\n", d1);
if (d1)
printf("node(%d) not found\n", d2);
}
return start;
}