How to drop columns from Spark DataFrame containing only null values using Scala

Question

I want to drop columns from DataFrame whose all values are null. Suppose I have a DataFrame -

 col1 | col2 | col3 | col4
----------------------------           
  1     null    C1    null
  2     null   null   null                 
 null   null   null   null
  3     null    C2    null  

Column col2 and col4 have all null values, so I have to drop those columns from DataFrame whose all values are null. So the final DataFrame would look like the following-

 col1 | col3  
--------------          
  1      C1    
  2     null                        
 null   null        
  3      C2     

The operation should take close to O(1).