linked-list-cycle——链表、判断是否循环链表、快慢指针

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

若在while开始时判断fast==slow，会出现误判，即第一次循环时fast必定等于slow

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12         if(head==NULL)
13             return false;
14         ListNode *slow=head,*fast=head;
15         while(fast!=NULL&&fast->next!=NULL){
16
17             fast=fast->next->next;
18             slow=slow->next;
19             if(fast==slow)
20                 return true;
21
22         }
23         return false;
24     }
25 };