public Categorie recalculerDerniereReponse(Reponse ancienneReponse) {
        if (ancienneReponse.equals(getDerniereReponse())) {
            derniereReponse = null;
            for (Sujet sujet : getSujets()) {
                if (getDerniereReponse() == null || sujet.getDerniereReponse().getTempsMillis() > getDerniereReponse().getTempsMillis()) {
                    this.derniereReponse = sujet.getDerniereReponse();
                }
            }
            if (getCategorieParent() == null)
                return this;
            Categorie cat = getCategorieParent().recalculerDerniereReponse(ancienneReponse);
            return cat != null ? cat : this;
        }
        return null;
    }