快慢指针（总会追上的）

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == nullptr || head->next == nullptr)
            return false;
        ListNode * slow = head, * fast = head->next;

        while (fast != nullptr && fast->next != nullptr)
        {
            if (fast == slow)
                return true;
            slow = slow->next;
            fast = fast->next->next;
        }

        return false;
    }
};