COMPLEJIDAD PREORDEN_ITERATIVO

                                                                              //O(n^2)
    private String preOrden_Iterativo(NodoBin<T> r){                            //T=2+2+2+E(2+1+4+2)+2+E(2+2+2+2+E(2+5+1+2))+1 = 9+17n+20n^2
        Pila<NodoBin> p=new Pila<NodoBin>();                                    //2
        String rr="";                                                           //2
        while(r!=null){                                                         //2+E(2)  
            p.apilar(r);                                                   //1
            rr+=r.getInfo().toString()+"-";                                     //4
            r=r.getIzq();                                                       //2
        }                       
        while(!p.esVacia()){                                                    //2+E(2)
            r=p.desapilar();                                                    //2
            r=r.getDer();                                                       //2
            while(r!=null){                                                     //2+E(2)
                rr+=r.getInfo().toString()+"-";                                 //5
                p.apilar(r);                                               //1
                r=r.getIzq();                                                   //2
            }
        }
        return(rr);                                                             //1
    }
T=2+2+2+\sum_{1=0}^{n}(2+1+4+2)+2+\sum_{1=0}^{n}(2+2+2+2+\sum_{1=0}^{n}(2+5+1+2))+1
T=9+17n+20n^2

T=20n^2

O(n^2)