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