public static String alinea5(MorseTree tree, Morse a , Morse b){
       String common="";
       return alinea5b(tree.root(), a,b,common);
        }



    private static String alinea5b(Node<Morse> node, Morse a, Morse b,String common){
        if(node==null
                ||node.getElement()==a
                ||node.getElement()==b
                ||( a.compareTo(node.getElement() ) ) != ( b.compareTo(node.getElement() ) )){
            return common;
        } else{
            if(a.compareTo(node.getElement())<0 ){
                common = common + ".";
                return alinea5b( node.getLeft(),a,b,common);
            }else{
                common = common + "_";
                return alinea5b( node.getRight(),a,b,common);
            }
        }