领扣——141环形链表（快慢指针）

给定一个链表，判断链表中是否有环。

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
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) {
    //只要有一个为空，则无环
        if (fast == null || fast.next == null) {
            return false;
        }
        //移动快慢指针
        slow = slow.next;
        fast = fast.next.next;
    }
    return true;
    }
}