title: Find a vertex that all other vertices can be reached from in a digraph
body: <p>Given a directed graph, how can we determine whether or not there exists a vertex v, from which all other vertices are reachable. the algorithm should be as efficient as possible.</p>
  
  <p>I know how to do it if we are checking for a given vertex; we could do dfs on the reverse graph. But for this question, it seems inefficient to do it for every vertex in the graph.</p>
  
  <p>Is there a better way?</p>