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