title: How to remove a map key when key and value are reseral
body: <p>everybody. I have a problem that I want to remove a key in a map when the map contains a reversal value, the code below:</p>
  <pre><code>Map&lt;Integer, Integer&gt; test = new HashMap&lt;&gt;();
  test.put(0, 1);
  test.put(1, 0);
  test.put(2, 1);
  test.put(3, 2);
  </code></pre>
  <p>IN the above code, you can see 0-1 and 1-0, but I just want to exist only one on the map like 0-1 or 1-0. Therefore I must remove one or more to keep only one stay on the map. In this case, the result should be like 0,1 and 2,1. I mean in this case, the result retains two-element on the map. Can someone help me that I very much appreciate it.</p>