判断银行卡号是否有效

//剔除卡号里的非法字符
-(NSString *)getDigitsOnly:(NSString*)s
{
    NSString *digitsOnly = @"";
    char c;
    for (int i = 0; i < s.length; i++)
    {
        c = [s characterAtIndex:i];
        if (isdigit(c))
        {
            digitsOnly =[digitsOnly stringByAppendingFormat:@"%c",c];
        }
    }
    return digitsOnly;
}

//检查银行卡是否合法
//Luhn算法
-(BOOL)isValidCardNumber:(NSString *)cardNumber
{
    NSString *digitsOnly = [self getDigitsOnly:cardNumber];
    int sum = 0;
    int digit = 0;
    int addend = 0;
    BOOL timesTwo = false;
    for (int i = digitsOnly.length - 1; i >= 0; i--)
    {
        digit = [digitsOnly characterAtIndex:i] - '0';
        if (timesTwo)
        {
            addend = digit * 2;
            if (addend > 9) {
                addend -= 9;
            }
        }
        else {
            addend = digit;
        }
        sum += addend;
        timesTwo = !timesTwo;
    }
    int modulus = sum % 10;
    return modulus == 0;
}