title: Should I use dict or list?
body: <p>I would like to loop through a big two dimension list:</p>
  
  <pre><code>authors = [["Bob", "Lisa"], ["Alice", "Bob"], ["Molly", "Jim"], ... ]
  </code></pre>
  
  <p>and get a list that contains all the names that occurs in authors.</p>
  
  <p>When I loop through the list, I need a container to store names I've already seen, I'm wondering if I should use a list or a dict:</p>
  
  <p>with a list:</p>
  
  <pre><code>seen = []
  for author_list in authors:
      for author in author_list:
          if not author in seen:
              seen.append(author)
  result = seen
  </code></pre>
  
  <p>with a dict:</p>
  
  <pre><code>seen = {}
  for author_list in authors:
      for author in author_list:
          if not author in seen:
              seen[author] = True
  result = seen.keys()
  </code></pre>
  
  <p>which one is faster? or is there better solutions?</p>