/* Definition for singly-linked list.
 struct ListNode {
     int val;
     struct ListNode *next;
 };
*/ 
 
bool hasCycle(struct ListNode *head) {
    if(head!=NULL)
    {
        struct ListNode *p=head->next;
        struct ListNode *q=p;
        while(p!=NULL&&q!=NULL&&q->next!=NULL)
        {
            p=p->next;
            q=q->next->next;
            if(p==q)
                return true;
        }
    }
    return false;
}