说明：  利用逆转换法，用均匀分布随机数产生指数随机数的数学实验，并绘制直方图检验效果,有兴趣的matlab初学者不妨看看，直方图绘制方法可推广借鉴
(mathematical experiment: using a random number to create exp distributed random index number, and drawing the histogram test the effect, catering to interested matlab beginner.)