c是在'0'-'9'之间，这样，c-'0'的取值范围是多少
'0'字符编码是48'9'字符编码是57
'0'-'0' 就等于 48-48 = 0'9'-'0' 就等于 57-48 = 9所以c-'0'的范围是0-9
0-9