title: Graphs: find a sink in less than O(|V|) - or show it can't be done
body: <p>I have a graph with <code>n</code> nodes as an <em>adjacency matrix</em>.</p>
  
  <p>Is it possible to detect a sink in less than <code>O(n)</code> time?</p>
  
  <p>If yes, how? If no, how do we prove it?</p>
  
  <p><strong>Sink vertex</strong> is a vertex that has incoming edges from other nodes and no outgoing edges.</p>