StopWatch

使用方法: StopWatch sw; sw.start(); ... // Ta double sec = sw.seconds(); // == Ta

sw.re_start();
... // Ta
sw.stop();
... 
sw.start();
...  // Tb
sec = sw.seconds(); // == Ta + Tb
kkk ,sec = sw.seconds(); // == Ta + Tb
sec = sw.seconds(); // == Ta + Tb