说明：  求一种快速生成随机数的算法，问题如下：从0~9、a-z中随机抽取18个字符组成一个18位随机数， 一共需要生成10亿个这样的随机数
(For a fast algorithm to generate random numbers, the problem is as follows: from 0 ~ 9, az in 18 randomly selected 18 characters to form a random number, generating a total of one billion such a random number)