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?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *slow = head,*fast=head;
        
        while(fast&&fast->next)
        {
            slow=slow->next;
            fast=fast->next->next;
            
            if(slow==fast)
            {
                ListNode *slow2=head;
                while(slow2!=slow)
                {
                    slow2=slow2->next;
                    slow=slow->next;
                    
                }
                return slow2;
            }
        }
        return NULL;
        
    }
};