Finding loop in a singly linked-list

Question

How can I detect that whether a singly linked-list has loop or not?? If it has loop then how to find the point of origination of the loop i.e. the node from which the loop

Answer 1

Following code will find whether there is a loop in SLL and if there, will return then starting node.

int find_loop(Node *head){

    Node * slow = head;
    Node * fast =  head;
    Node * ptr1;
    Node * ptr2;
    int k =1, loop_found =0, i;

    if(!head) return -1;

    while(slow && fast && fast->next){
            slow = slow->next;
        /*Moving fast pointer two steps at a time */
            fast = fast->next->next;
            if(slow == fast){
                    loop_found = 1;
                    break;
            }

    }

    if(loop_found){
    /* We have detected a loop */
    /*Let's count the number of nodes in this loop node */

            ptr1  = fast;
            while(ptr1 && ptr1->next != slow){
                    ptr1 = ptr1->next;
                    k++;
            }
    /* Now move the other pointer by K nodes */
            ptr2 = head;

            ptr1  = head;
            for(i=0; inext;
            }

    /* Now if we move ptr1 and ptr2 with same speed they will meet at start of loop */

            while(ptr1 != ptr2){
                    ptr1  = ptr1->next;
                    ptr2 =  ptr2->next;
            }

    return ptr1->data;

}