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 };