if (48 <= (int)a[i] <= 57)和if（48 <= (int)a[i]&&a[i] <= 57)有什么区别
48 <= (int)a[i] <= 57是48 <= (int)a[i]的结果（1或0）再和57比较，而48 <= (int)a[i]&&a[i] <= 57是a[i]和48、57比较