请设计程序，使其实现以秒计时的功能。首先定义一个watch类，它有两个私有变量begin、end分别表示开始时间、结束时间，有成员函数start()、stop()、show()分别用来设置开始时间、结束时间、显示持续时间。-requested the design process to enable them to function seconds time. First define a watch category, which has two private variables begin and end, said start time, ending time, a member function start () and stop (), the show () respectively for the establishment of start time, the end of time, showing the duration.