title: stack all elements of list in R (including list in list)
body: <p>consider</p>
  
  <pre><code>mylist=list('a',c('aa','bb','cc'),'d')
  </code></pre>
  
  <p>I have two questions(I want to stack them in two ways):
  First:
  I want column output like</p>
  
  <pre><code>col1
  a
  aa
  bb
  cc
  d
  </code></pre>
  
  <p>Second:
  I want dataframe like</p>
  
  <pre><code>col1 col2 col3
  a    NA   NA 
  aa   bb   cc
  d    NA   NA
  </code></pre>