title: Merging lists into a single array with unique elements
body: <p>I have five lists <code>List&lt;String&gt;</code>. I want to get a single array which contains all elements from those lists, eliminating any repartitions. What could be the best way to do this?</p>
  
  <p><strong>`Edit: Can someone also comment on the performance of HashSet in comparision to List. I am concerned about performance, as I am doing this job while calculating the data to be displayed on a webpage. And no. of elements in the set would be high around 300-400, what parameters whould be suitable for the Set?</strong></p>
  
  <p>My elements in set, would be of this type:
  <code>&lt;HColumn&lt;String, String&gt;&gt;</code></p>