/**
 * 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 head;
        ListNode *low = head;
        ListNode *fast = head->next;
        while (fast && fast->next) {
            if (low->val == fast->val) return true;
            low = low->next;
            fast = fast->next->next;
        }
        return false;
    }
};