iOS-登录加密也许用到，反转字符串

- (NSString *)stringByReversed
{
//    NSMutableString *s = [NSMutableString string];
//    for (NSUInteger i=self.length; i>0; i--) {
//        [s appendString:[self substringWithRange:NSMakeRange(i-1, 1)]];
//    }
//    return s;
    unsigned long len;
    len = [self length];
    unichar a[len];
    for(int i = 0; i < len; i++)
    {
        unichar c = [self characterAtIndex:len-i-1];
        a[i] = c;
    }
    NSString *str1=[NSString stringWithCharacters:a length:len];
    return  str1;
}