title: how do you find all the nodes that are reachable in a graph
body: <p>Given a node in a graph that is reachable, how do you find all the nodes that are reachable ? How would you enable parallel computation of this information ? Given a number of cores, how many threads would you choose ?</p>
  
  <p>For the first part, i would go with the Breadth first Search and second part, i think somehow i should use the strnegth of Map-reduce but not able to understand how? 
  Could you please help. Appreciate your time and efforts.</p>