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) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
ListNode * itr = head;
int n = 1;
while (true){
if (itr==NULL)
return false;
ListNode * tmp = itr;
for (int i=0; i<n; i++){
tmp = tmp->next;
if (tmp==NULL)
return false;
else if (tmp == itr)
return true;
}
n++;
itr = itr->next;
}
return false;
}
};