title: what are the relationship between O(nlogn)+O(n), O(nlogn) and O(nlogn + n)?
body: <p>Intuitively, I thought the three expressions are equivalent.</p>
  
  <p>For example, if an algorithm runs in <code>O(nlogn) + O(n)</code> or <code>O(nlogn + n)</code>(I'm confused), can I say that's an <code>O(nlogn)</code> algorithm?</p>
  
  <p>what's the truth?</p>