title: Interview Question: Reverse pairs
body: <p>I got this for my interview:</p>
  
  <p>Numbers are said to be "reverse ordered" if N[i] > N[j] for i &lt; j.
  For example, in a list: 3 4 1 6 7 3, the reverse ordered items are (3,1) (4,1) (4,3) (6,3) (7,3). </p>
  
  <p>How to get the number of pairs of reverse ordered items in O(nlogn) time.</p>