141. 环形链表

public class Solution {
    public boolean hasCycle(ListNode head) {
       if (head==null||head.next==null){
            return false;
        }
        ListNode slow=head;
        ListNode fast = head.next;

        while (slow!=fast){
            if (fast==null||fast.next==null){//这里注意空指针异常
                return false;
            }
            slow=slow.next;
            fast=fast.next.next;
        }
        return true;
    }
}