1.题目：​​https://leetcode-cn.com/problems/linked-list-cycle/​​

2.思路：

（1）双指针（快慢指针）来判断是否是环路
（2）哈希思想：设成一个不会出现的特殊值就行了，重点是在原数据结构上标记的思想

3.代码：

参考：https://leetcode-cn.com/problems/linked-list-cycle/solution/kuai-man-zhi-zhen-da-jia-jian-de-duo-liao-lai-ge-z/
https://leetcode-cn.com/problems/linked-list-cycle/comments/97300
方法1
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head==NULL)
            return false;
        ListNode *fast= head;
        ListNode *slow= head;
        while (fast!=NULL && fast->next!=NULL)
        {
            fast=fast->next->next;
            slow=slow->next;
            if (fast==slow)
                return true;
        }
        return false;
        
    }
};
方法2：
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head==NULL)
            return false;
        ListNode *fast=head;
        while (fast!=NULL && fast->val!=99999999)
        {
            fast->val=99999999;
            fast=fast->next;
        }
        if (fast!=NULL)
            return true;
        else
            return false;
        
    }
};