微信和支付宝支付的金额不能超过两位小数，可以把后台超过两位小数的支付字符串修正为两位小数

-(NSString *)modifyWithStr:(NSString *)str
{
    if(isCommonUnitEmptyString(str))
    {
        return @"";
    }
    
    NSInteger flag = 0;//0开始，没有找到值；1找到值但是没有找全值；2找全值，在找剩余右边的字符串
    NSString *actualAmountStr = @"";
    NSString *actualAmountValueStr = @"";
    NSString *rightStr = @"";
    NSString *newStr = @"";
    if(!isCommonUnitEmpty(str) && str.length > 8)
    {
        for(NSInteger n =0; n < str.length; n++)
        {
            NSString *str1 = [str substringWithRange:NSMakeRange(n, 1)];
            if(!isCommonUnitEmptyString(str1))
            {
                if(0 == flag)
                {
                    if([str1 isEqualToString:@"="])
                    {
                        flag = 1;
                    }
                    actualAmountStr = [NSString stringWithFormat:@"%@%@", actualAmountStr, str1];
                }
                else if(1 == flag)
                {
                    if([str1 isEqualToString:@"&"])
                    {
                        rightStr = @"&";
                        flag = 2;
                    }
                    else
                    {
                        actualAmountValueStr = [NSString stringWithFormat:@"%@%@", actualAmountValueStr, str1];
                    }
                }
                else
                {
                    rightStr = [NSString stringWithFormat:@"%@%@", rightStr, str1];
                }
            }
            else
            {
                break;
            }
        }
        if(2 ==flag)
        {
            CGFloat actualAmountValue = [actualAmountValueStr floatValue] +0.005;
            newStr = [NSString stringWithFormat:@"%@%.2f%@", actualAmountStr, actualAmountValue,rightStr];
        }
    }
    return newStr;
}

//判断是否为空
static inline BOOL isCommonUnitEmpty(id thing) {
    return thing == nil ||
    ([thing isEqual:[NSNull null]]) ||
    ([thing respondsToSelector:@selector(length)] && [(NSData *)thing length] == 0) || [([NSString stringWithFormat:@"%@", thing]) isEqualToString:@"<null>"] ||
    ([thing respondsToSelector:@selector(count)]  && [(NSArray *)thing count] == 0);
}