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) {
ListNode *fast, *slow;
if (head == NULL || head->next == NULL) {
return false;
}

slow = head;
fast = head->next->next;

while (fast && fast->next) {
if (slow == fast) {
return true;
}
slow = slow->next;
fast = fast->next->next;
}

return false;
}
};