leetcode 141. linked list cycle

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