title: How to copy value of two sub list into a single list in c#?
body: <p>I have a collection of list </p>
  
  <pre><code>List&lt;List&lt;int&gt;&gt; FS=new List&lt;List&lt;int&gt;&gt;()
  </code></pre>
  
  <p>I've added Values to each list. Now my <code>FS</code> Look like <code>FS={{1,2,3},{5,7},{3}}</code>.</p>
  
  <p>How to combine the values to first two sub list into a single new list?
  my need to get like <code>{1,2,3,5,7}</code></p>