LeetCode：Linked List Cycle

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

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

 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         ListNode *slow=head;
13         ListNode *fast=head;
14
15         while(fast&&fast->next)
16         {
17             slow=slow->next;
18             fast=fast->next->next;
19             if(slow==fast) return true;
20         }
21         return false;
22
23     }
24 };