title: Convert nested list into dataframe
body: <p>I saw similar questions answered in stackoverflow, but I couldn't find the way to fit my specific situation, so I open a new one.</p>
  
  <p>My goal is to convert a nested list like this:</p>
  
  <pre><code>a:list of 2
  ..$ time: num[1:5] 1 2 3 4 5
  ..$ int: num[1:5] 6 7 8 9 0
  
  b:list of 2
  ..$ time: num[1:5] 1 2 3 4 5
  ..$ int: num[1:5] 6 7 8 9 0
  </code></pre>
  
  <p>into a dataframe like this:</p>
  
  <pre><code>Name time int
  a    1    6
  a    2    7
  .
  .
  .
  b    1    6
  ...
  </code></pre>
  
  <p>I tried to use <code>melt</code> in <code>reshape2</code> and I got time and int as the values in one column(L2). Something like this:</p>
  
  <pre><code>Value L2    L1
  1     time  a
  2     time  a
  .
  .
  .
  6     int   a
  .
  .
  .
  </code></pre>