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?

代码：

/**
 * 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 *oneStep = head;
		ListNode *twoStep = head;
		while (oneStep && twoStep->next) {
			oneStep = oneStep->next;
			twoStep = twoStep->next->next;
			if (oneStep == NULL || twoStep == NULL) {
				return false;
			}

			if (oneStep == twoStep) { 
				return true;
			}
		}
		return false;
    }
};