title: Python: given a list of lists, create a list ordered by the number of occurrences in inner list
body: <p>I have a list of lists:</p>
  
  <pre><code>[['a','b','c'], ['a'], ['a','b']]
  </code></pre>
  
  <p>I want to sort it and return a single list so the output looks like this:</p>
  
  <pre><code>['a', 'b', 'c']
  </code></pre>
  
  <p>i.e. ordered by the number of times each element appears. a appears 3 times, b appears twice, and c appears once. </p>
  
  <p>How do I go about doing this?</p>