title: Invert Map with list value Map<Key, List<Value>> to Map <Value, Key> in Java 8
body: <p>I have a map kind of grouping values by key <code>Map&lt;String, List&lt;Integer&gt;&gt;</code>, i want to revert in order to map each value to the corresponding key</p>
  
  <p>Example: I want to transform the code below </p>
  
  <pre><code>Map&lt;String, List&lt;Integer&gt;&gt; mapOfIntList = new HashMap&lt;String, List&lt;Integer&gt;&gt;();
  
  mapOfIntList.put("UNIT", Arrays.asList(1, 2, 3, 8, 7, 0, 8, 6));
  mapOfIntList.put("TEN", Arrays.asList(24, 90, 63, 87));
  mapOfIntList.put("HUNDRED", Arrays.asList(645, 457, 306, 762));
  mapOfIntList.put("THOUSAND", Arrays.asList(1234, 3456, 5340, 9876));
  </code></pre>
  
  <p>to another Map(Integer, String) where i can find :
  (1, "UNIT"), (2, "UNIT")...(24, "TEN"), (90, "TEN")...(645, "HUNDRED")...(3456, "THOUSAND")...</p>