title: Count of elements in lists within pandas data frame
body: <p>I need to get the frequency of each element in a list when the list is in a pandas data frame columns </p>
  
  <p>In data:</p>
  
  <pre><code>din=pd.DataFrame({'x':[['a','b','c'],['a','e','d', 'c']]})`
  
                x
  0     [a, b, c]
  1  [a, e, d, c]
  </code></pre>
  
  <p>Desired Output:</p>
  
  <pre><code>   f  x
  0  2  a
  1  1  b
  2  2  c
  3  1  d
  4  1  e
  </code></pre>
  
  <p>I can expand the list into rows and then perform a group by but this data could be large ( million plus records ) and was wondering if there is a more efficient/direct way.</p>
  
  <p>Thanks</p>