双指针（快慢双指针）

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null)return false;
        ListNode t1 = head;//慢指针
        ListNode t2 = head;//快指针
        while(t2 != null && t2.next != null){
            t1 = t1.next;
            t2 = t2.next.next;
            if(t1 == t2)return true;
        }
        return false;
    }
}

暴力

观察数据，发现节点最多10000个，直接循环计数，循环的次数大于10000即代表出现了环

public class Solution {
    public boolean hasCycle(ListNode head) {
        //记录循环的次数
        int ret = 0;
        while(head != null){
            ret++;
            head = head.next;
            if(ret > 11000){
                break;
            }
        }
        return ret > 11000;
    }
}