141. 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* p1 = head, *p2 = head;
        while(true){
            if(NULL == p2 || NULL == p1) return false;
            p1 = p1->next;
            p2 = p2->next;
            if(NULL == p2) return false;
            p2 = p2->next;
            if(p2 == p1) return true;
        }
    }
};