struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(nullptr) {}
};

bool hasCycle(ListNode *head) {
    if (!head || !head->next) return false;

    ListNode *slow = head;
    ListNode *fast = head;

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

#include <unordered_set>

bool hasCycle(ListNode *head) {
    std::unordered_set<ListNode*> visited;
    while (head) {
        if (visited.find(head) != visited.end()) {
            return true;
        }
        visited.insert(head);
        head = head->next;
    }
    return false;
}

bool hasCycle(ListNode *head) {
    while (head) {
        if (head->next == head) return true; // 已被标记
        ListNode *next = head->next;
        head->next = head; // 标记当前节点
        head = next;
    }
    return false;
}