title: MapReduce - How sort reduce output by value
body: <p>How can I sort in decreasing order the reducer output by value?
  I'm developing an application that must return top listened songs. Thus songs must be ordered by the number of listening.
  My application works in this way:</p>
  
  <pre><code>Input: songname@userid@boolean
  MapOutput : songname userid
  ReduceOutput : songname number_of_listening
  </code></pre>
  
  <p>Any idea how to do this?</p>