数据结构与算法之Stack（栈）的应用——用stack实现一个计算器-/bin/calc.dart

计算器的bin/calc.dart 可执行代码：

 import 'dart:io';
 import 'package:data_struct/stack/sample/calculator.dart';

 void main() {
   print('please input the expression:');
   var ins = stdin.readLineSync();
   while (ins != 'exit') {
     try {
       var r = calc(ins);
       print('\n the result is: $r');
     } on FormatException {
       print('Format error!');
     } on Exception catch (e) {
       print('Unknown error: $e!');
     }

     print('\nplease input the expression:');
     ins = stdin.readLineSync();
   }
 }