Node* next(Node* node) {
    if (node)
        node = node->next;
    return node;
}

bool has_cycle(Node* head) {
    Node* slow = next(head);
    Node* fast = next(next(head));
    
    while (slow != fast) {
        slow = next(slow);
        fast = next(next(fast));
    }
    
    return fast;
}