typedef struct listNode {
    int val;
    struct listNode *next;
} listNode;

bool hasCycle(struct ListNode *head) {
    if (head == NULL) return false;
    listNode *fast = head->next, *slow = head;
    while (fast != NULL && fast->next != NULL && fast->next->next != NULL) {
        if (fast == slow) {
            return true;
        }
        fast = fast->next->next;
        slow = slow->next;
    }
    return false;
}