Leetcode -- Linked List Cycle

/**  
  * 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) {  
       return false;  
     }  
     ListNode * dummy = new ListNode(-1);  
     dummy->next = head;  
     ListNode * p = dummy;  
     ListNode * q = dummy;  
     p = p->next;  
     q = q->next;  
     q = q->next;  
     while(p != NULL && q != NULL && q->next != NULL) {  
       if(p == q) {  
         return true;  
       }  
       p = p->next;  
       q = q->next;  
       q= q->next;  
     }  
     return false;  
   }  
 };