title: Interval selection algorithm
body: <p>Let's say we have a set of intervals
  <code>[s1,e1],[s2,e2]...[sn,en]</code></p>
  
  <p>I would like to find the subset of non-overlapping intervals and has the maximum aggregate time.</p>
  
  <p>Actually I'm looking for a greedy solution. Does it exist or not?</p>