Linked List Cycle [Leetcode]

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

Solution: classic interview question, which can be solve by slow-fast pointers. The speed of fast pointer is twice of speed of slow one. If a cycle exists, two pointers would meet somewhere. [here is a follow-up question]

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(!head || !head->next || !head->next->next)
            return false;
        ListNode* slow = head->next;
        ListNode* fast = head->next->next;
        while(slow!=fast){
            if(!fast->next || !fast->next->next)
                return false;
            slow=slow->next;
            fast=fast->next->next;
        }
        return true;
    }
};