字符串转为unicode

+ (NSString *)utf8ToUnicode:(NSString *)string{

    NSUInteger length = [stringlength];

    NSMutableString *s = [NSMutableStringstringWithCapacity:0];

    for (int i =0;i < length; i++){

        unichar _char = [stringcharacterAtIndex:i];

        //判断是否为英文和数字

        if (_char <='9' && _char >='0'){

            [s appendFormat:@"%@",[stringsubstringWithRange:NSMakeRange(i,1)]];

        }elseif(_char >='a' && _char <='z'){

            [s appendFormat:@"%@",[stringsubstringWithRange:NSMakeRange(i,1)]];

        }elseif(_char >='A' && _char <='Z'){

            [s appendFormat:@"%@",[stringsubstringWithRange:NSMakeRange(i,1)]];

        }else{

            

            NSString * str = [NSStringstringWithFormat:@"\\u%x",[stringcharacterAtIndex:i]];

            if ([strrangeOfString:@"ud"].location!=NSNotFound) {

                [s appendFormat:@"\\u%x",[stringcharacterAtIndex:i]];

            }else{

                [s appendFormat:@"%@",[stringsubstringWithRange:NSMakeRange(i,1)]];

            }

        }

    }

    if (s) {

        return s;

    }elseif (string && [stringisKindOfClass:[NSStringclass]]){

        return string;

    }

    return@"***";

}