title: which data structure to use for giving unique riddles to users?
body: <p>So I am making a website where a group of users try to answer riddles at the same time and later are able to submit riddles to the website. </p>
  
  <p>I want to make it so that a user is never given the same riddle twice. So I wanted advice on how to create a User.js class and a Group.js class</p>
  
  <p>User.js class needs a data structure to represent riddles it's never seen. Let's call it RiddlesNotSeenByUser for now.</p>
  
  <p>The Group.js class needs to be able to merge all the RiddlesNotSeenByUser for each user into a data structure let's call RiddlesNotSeenByGroup. </p>
  
  <p>Then I am going to have a huge database of riddles and I am going to need to efficiently
  find a riddle in the database not in the data structure RiddlesNotSeenByGroup.</p>
  
  <p>Thanks!</p>