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 *pHead = head, *pAfter = head;
		while (pHead != NULL && pHead->next != NULL) {
			pHead = pHead->next->next;
			pAfter = pAfter->next;

			if (pHead == pAfter)
				break;
		}

		if (pHead == pAfter)
			return true;
		else
			return false;
	}
};