title: C++ get the difference between two vectors
body: <p>imagine you got 2 vectors:</p>
  
  <pre><code>vector&lt;int&gt; ar1, a2;
  
  ar1 = {1,1,2,3,3,4,5,5,6};
  ar2 = {1,2,3,4,5,6};
  </code></pre>
  
  <p>how to do something like this in a good way (using C++) ?</p>
  
  <pre><code>b = ar1 - ar2
  // b = {1,3,5}
  </code></pre>