LeetCode--Linked List Cycle II

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

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) 
    {
        if(head==NULL || head->next==NULL)
            return NULL;
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast!=NULL && fast->next!=NULL)
        {
            slow = slow->next;
            fast = fast->next;
            fast = fast->next;
            if(slow == fast)
                break;
        }
        if(slow == fast)
        {
            slow = head;
            while(slow!=fast)
            {
                slow = slow->next;
                fast = fast->next;
            }
            return slow;
        }
        return NULL;
    }
};