public class Solution {
    //n 1
    public boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast!= null && fast.next!= null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }
}