title: Convert a HashMap<Integer, List<String>> to HashMap<String, HashSet<Integer>>
body: <p>I have a <code>HashMap&lt;Integer, List&lt;String&gt;&gt;</code> as input,  e.g : </p>
  
  <pre><code>3  :  a, b, c
  6  :  b, c
  9  :  a, c
  12 :  b ,c
  </code></pre>
  
  <p>I want to convert it to <code>HashMap&lt;String, HashSet&lt;Integer&gt;&gt;</code> for e.g.</p>
  
  <pre><code>a : 3, 9
  b : 3, 6, 12
  c : 3, 6, 9, 12
  </code></pre>
  
  <p>How can it be done ? </p>