ViewHolder.java
public class ViewHolder extends RecyclerView.ViewHolder {
    public TextView list_title;
    public TextView list_desc;
    public ViewHolder(View itemView) {
        super(itemView);
        list_title = (TextView) itemView.findViewById(R.id.list_title);
        list_desc = (TextView) itemView.findViewById(R.id.list_desc);
    }
}
에서 itemView.findViewById(R.id.list_desc)랑 itemView.findViewById(R.id.list_title)에서 Cannot resolve symbol 'R'로 에러가 떠요..
list_item.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <TextView
        android:id="@+id/list_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="리스트 화면"
        android:textColor="#fff"
        android:background="#36384F"
        android:textSize="20sp"
        android:paddingLeft="20dp"
        android:paddingTop="20dp"
        android:paddingBottom="10dp"/>
    <TextView
        android:id="@+id/list_desc"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="리스트 내용"
        android:textColor="#fff"
        android:background="#36384F"
        android:textSize="15sp"
        android:maxLines="2"
        android:ellipsize="end"
        android:paddingLeft="20dp"
        android:paddingBottom="20dp"/>
</LinearLayout>