In Excel, how do you auto fill until next value?

currently I have a column looks like this:

  A B ...
1 A
2
3
4 B
5 
6 
7 C
8 
...

And I want to make it looks like this

  A B ...
1 A
2 A
3 A
4 B
5 B
6 B
7 C
8 C
...

Is there a way to finish the job quickly? Thanks a lot!