String drukujDrzewo(Tree t, int l){
		String s = "";
		if(t != null){
			s += drukujDrzewo(t.lt, l+1);
			for(int i = 0; i < l; i++) s +="  ";
			s += t.op+"\n";
			s += drukujDrzewo(t.pt, l+1);
 		}
		return s;
	}