+ (BOOL)checkCreditCardNumber:(NSString *)cardNum
{
    NSAssert(cardNum && cardNum != @"" , @"checkCreditCardNumber: cardNum is nil.");
    
    NSInteger len = [cardNum length];
    NSInteger sumNumOdd = 0;
    NSInteger sumNumEven = 0;
    BOOL isOdd = YES;
        
    for (NSInteger i = len - 1; i >= 0; i--) {
        
        NSInteger num = [cardNum substringWithRange:NSMakeRange(i, 1)].integerValue;
        if (isOdd) {//奇数位
            sumNumOdd += num;
        }else{//偶数位
            num = num * 2;
            if (num > 9) {
                num = num - 9;
            }
            sumNumEven += num;
        }
         isOdd = !isOdd;
    }
    
    return ((sumNumOdd + sumNumEven) % 10 == 0);
}
//objectc/7096