title: Reverse many-to-many Dictionary<key, List<value>>
body: <p>Actually <a href="https://stackoverflow.com/questions/1324912/convert-dictionaryint-enumerable-to-dictionaryint-enumerable-inverting-cont">my previous question</a> got me thinking
  and I realized that reversing a <code>Dictionary</code> is not trivial.
  What is the most elegant and readable way to do it?</p>
  
  <p>Same scenario student Many to Many with Classes</p>
  
  <p>original <code>Dicitonary&lt;int, List&lt;int&gt;&gt;</code> where the key is studentId and the Value is a <code>List&lt;int&gt;</code> that contains classId and want to revert to <code>Dictionary&lt;classId, List&lt;studentId&gt;&gt;</code></p>
  
  <p>Thanks</p>
  
  <p>Update:
  Actually I just tested Luke and Bruno's solutions and they return the right amount of classed whoever they all have the same student, will update as i go along.</p>