Leetcode77: 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* fast, *slow;
if(!head)
return false;
fast = head->next;
slow = head;
while(fast != NULL && fast->next != NULL)
{
if(fast == slow)
return true;
fast = fast->next->next;
slow = slow->next;
}
return false;
}
};