bool has_cycle(Node* head) {
    // Complete this function
    // Do not write the main method
    
    Node* slowerPtr = head;
    Node* fasterPtr = head;
    
    if ( (head == nullptr) || (head->next == nullptr) ){
        return false;   
    }
    
    while( (fasterPtr != nullptr) && (fasterPtr->next != nullptr) ){
        slowerPtr = slowerPtr->next;
        fasterPtr = fasterPtr->next->next;
        if (slowerPtr == fasterPtr){
            return true;
        }
    }
    return false;
}