LeetCode:Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

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     ListNode *detectCycle(ListNode *head) {
12         ListNode *slow = head,*fast=head;
13         
14         while(fast&&fast->next)
15         {
16             slow=slow->next;
17             fast=fast->next->next;
18             
19             if(slow==fast)
20             {
21                 ListNode *slow2=head;
22                 while(slow2!=slow)
23                 {
24                     slow2=slow2->next;
25                     slow=slow->next;
26                     
27                 }
28                 return slow2;
29             }
30         }
31         return NULL;
32         
33     }
34 };