leetcode 141.环形链表

题目描述：

给定一个链表，判断链表中是否有环。

代码：

// 快慢指针
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) return false;
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }
}