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* p=head->next;
map<ListNode*,int> r;
r[head]=1;
while(p){
if(r[p]==1) return true;
r[p]=1;
p=p->next;
}
return false;
}
};