public ListNode detectCycle(ListNode head) {
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null)
        {
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow)
                break;
        }
        if(fast==null||fast.next==null)
         return null;
        fast=head;
        while(fast!=slow)
        {
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }