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