assumption: there is a value which was never been used in every node. (let's say INT_MIN)

solution: move node by node and set INT_MIN for the node visited. if a node has INT_MIN value, the given linked list has a 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) return false;
ListNode *node = head;
while(node->next){
if(node->val == INT_MIN){
return true;
}
node->val = INT_MIN;
node = node->next;
}
return false;
}
};
```