if (stack.Count > 0)
                    {
                        char c2 = stack.Peek()[0];

                        while ((stack.Count > 0) && isOperator(c2) &&
                            (((c == '+') || (c == '-')) &&
                             ((c2 == '*') || (c2 == '/'))))
                        {
                            postfix.Push(stack.Pop());
                            if (stack.Count > 0)
                            {
                                c2 = stack.Peek()[0];
                            }
                        }
                    }