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?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *slow, *fast;
        slow = fast = head;
        while(true){
            if(slow==NULL || fast==NULL || slow->next==NULL || fast->next==NULL){
                return false;
            }
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){
                //has a cycle
                return true;
            }
        }
    }
};