#算法
校验信用卡号码用到的Luhn 算法，我试了一下中国的信用卡用的也是算法。
图片里的第二步“add the digits of the product(e.g. 1+6=7)”，也可以这么做：如果第一步得到的数大于9，说明是一个二位数，这时候需要减去9重新变成一位数。
BY codedump的电报频道
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