mark info of head :-will keep the values indicating the value that a each unvisited node will contain for a single traversing.( this value will toggle for alternate traversals.)

mark info of other node:- will contain the information that node is visited or not. to get cycle:- we will start with head node get the value of mark info and store in a temporary variable.
This value is the value that a node must contain if it is not visited in this traversal of list. then we toggle the mark value of head (in next traversal this value will be the test for un travesed nodes).

now we will visit rest node and check if this value is same as that of temporary variable. if (yes) then this is node that is not visited we will toggle mark info of node and visit next node. if (no) this node is already visited so there is cycle.

Take 2 pointer. increment 1 pointer once and another pointer twice.
keep incrementing unless pointer2 is NULL or when pointer 1 and pointer 2 are equal.

When both pointer equal there is a Loop existing.
otherwise no Loop Detected.

/* Source Code */
/* Assume that there is a linked list which is already existing */

void findLoop()
{
struct llist *ptr1 *ptr2;

ptr1 head;
ptr2 head->link;

if(ptr1 ptr2)
printf("nLoop Detectedn");
else
{
while((ptr2 ! NULL) && (ptr1 ! ptr2))
{
ptr1 ptr1->link;
ptr2 ptr2->link;
if(ptr2 ! NULL)
ptr2 ptr2->link;
}
if(ptr2 NULL)
printf("Loop is not Detectedn");
else
{
if(ptr1 ptr2)
{
printf("Loop Detectedn");
}
}
}

}