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