function ccc(List: TStringList; Index1, Index2: Integer): Integer;
begin
   if list[index1]>list[index2] then result:=-1
   else if list[index1]=list[index2] then result:=0
   else result:=1;
end;
procedure TForm1.Button1Click(Sender: TObject);
var s:tstringlist;
begin
  s:=tstringlist.create;
  s.add(a);
  s.add(c);
  s.add(b);
  s.CustomSort(ccc);
  memo1.lines:=s;
  s.free;
end;