Vim delete blank lines

Question

What command can I run to remove blank lines in Vim?

Answer 1

1. how to remove all the blanks lines

   :%s,\n\n,^M,g
   

   (do this multiple times util all the empty lines went gone)

2. how to remove all the blanks lines leaving SINGLE empty line

   :%s,\n\n\n,^M^M,g
   

   (do this multiple times)

3. how to remove all the blanks lines leaving TWO empty lines AT MAXIMUM,

   :%s,\n\n\n\n,^M^M^M,g
   

   (do this multiple times)

in order to input ^M, I have to control-Q and control-M in windows