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;

    }

};