CareerCup

public boolean hasCycle(ListNode head) {
	ListNode fast = head, slow = head;
	while (fast != null && fast.next != null) {
		fast = fast.next.next;
		slow = slow.next;
		if (fast == slow) return true;
	}
	return false;
}

Use two pointers : slow and fast. Slow pointer moves forward by one step and fast by two steps. If your linked list does not have a cycle then your fast will reach the end of linked list and the check will terminate. If there is a cycle, then fast and slow pointer will collide at some point. So, if fast == slow then there is a cycle in the linkedlist.

github.com/techpanja/interviewproblems/blob/master/src/collections/customlinkedlist/LinkedListImpl.java

/* Function returns 1 if it has a cycle and 0 if it doesn't has a cycle*/
int hashCycle(Node *head)
{
Node *fastNode = head;
while(head !=null)
{
if(head->next == fastNode)
return 1;
head=head->next;
}
return 0;
}

steps:
1)Fist create 2 pointer
slow pointr and fast pointer
2) both point to the starting node
Node *slowPtr=Start;
Node *FastPtr=Start;
3)now forward slow pointer by one step and fast pointer by 2 step
while(Start!=NULL)
{
FastPtr=Fast->next->next
SlowPtr=slow->next
if(SlowPtr==FastPtr)
return 1;
}
4 exit

Node{*nextNode, color='BLACK'}

Node *p = head;
Node *q = p;

while(p -> next != NULL){
q = p->next;
if(q->color == "WHITE"){
printf("Loop found");
break;
}else{
q->color = "WHITE";
}
}

read floyd cycle detection algo.

while (cur != NULL) {
   if (cur->data == "cycle") {
       return true;
  }
  cur = cur->next;
}
return false;