Merge sparse questions in multiple columns from questionnaire – Pandas

I have a pandas dataframe like this:

Q1 | Q2 | Q3 | Q4
   | 2  |    | 
1  |    |    |
   |    | 3  | 
   | 2  |    |
   |    |    | 4  

My desired output is:

Q
_
2
1
3
2
4
 

How can I do it easily in Pandas?

>Solution :

You could use stack

df.stack().reset_index(drop=True).to_frame().rename({0:'Q'},axis=1)

    Q
0  2.0
1  1.0
2  3.0
3  2.0
4  4.0