leetcode- 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==NULL||head->next==NULL)
        {
            return false;
        }
        ListNode *fast=head->next->next, *low=head;
        while(fast!=NULL)
        {
            if(fast==low)
            {
                return true;
            }
            else
            {
                fast = fast->next;
                if(fast==NULL)
                {
                    break;
                }
                else
                {
                    fast = fast->next;
                }
                low =low->next;
            }
        }
        return false;
    }
};