判断链表是不是循环链表

#include<iostream>
#include<vector>
using namespace std;
/*
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?*/
struct ListNode {int val;ListNode *next;ListNode(int x) : val(x), next(NULL) {}
};
class Solution {  //我的思路 错误
public:bool hasCycle(ListNode *head) {ListNode* faster = head;ListNode* slower = head;while (faster->next->next != NULL && faster->next != NULL) {faster = faster->next->next;slower = slower->next;}if (faster==slower)return true;return false;}
};
class Solution {
public:bool hasCycle(ListNode *head) {if (head == NULL)return false;ListNode* faster = head;ListNode* slower = head;while(faster->next->next != NULL && faster->next != NULL) {slower = slower->next;faster = faster->next->next;if (faster == slower)return true;}return false;}
};