/** * 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) { //只有一个无法成环 if(head==null||head.next==null)return false; ListNode node1=head; ListNode node2=head; //如果不成环,跳出循环 while(node2!=null&&node2.next!=null){ //确保可以追上 node1=node1.next; node2=node2.next.next; if(node1==node2)return true; } return false; } }