Filling blank field with previous row value in PL/SQL

Suppose I have Table named emp like this

empno	comm
033001	RUET
033002
033003
033004	KUET
033004
033005	CUET

Now I want to fill the blank cells with the previous value. Such as comm of 033002, 033003  will be RUET. comm of 033004 will be KUET. How can I do this? I had googled and found an interesting query.
1. I have to execute following query:
select empno,comm, lag(comm ignore nulls) over (order by empno)prev_comm from emp order by empno.
Then I get a view like this :

empno	comm	prev_comm
033001	RUET
033002		RUET
033003		RUET
033004	KUET	RUET
033004		KUET
033005	CUET	KUET
		CUET

2. Suppose the previous view is represented by mm.
Then I have to write the following query on mm view.
select empno, case when comm is null then prev_comm else comm end as comm from mm.
Then we will get the desired view :

empno	comm
033001	RUET
033002	RUET
033003	RUET
033004	KUET
033004	KUET
033005	CUET

So, the final query will be:
select empno, case when comm is null then prev_comm else comm end as comm from (select empno,comm, lag(comm ignore nulls) over (order by empno)prev_comm from emp order by empno)