Linked List Cycle II

class Solution {
public:
    ListNode *detectCycle(ListNode *head)
	{
		ListNode* slow = head;
		ListNode* fast = head;

		while (NULL != fast && NULL != fast->next)
		{
			slow = slow->next;
			fast = fast->next->next;

			if (slow == fast)
			{
				slow = head;
				while(slow != fast)
				{
					slow = slow->next;
					fast = fast->next;
				}
				return slow;
			}
		}
		return NULL;
    }
};

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?