题解 | #判断链表中是否有环#

import java.util.*;
/**
 * 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) {
            return false;
        }
        if (head.val == 100001) {
            return true;
        }
        head.val = 100001;
        return hasCycle(head.next);
    }
}

遍历链表，走过设置一个不可能存在的值，直到遍历到这个值返回true，遍历到尾返回false