title: How many BFS orderings are there?
body: <p>Given a graph <em>G(V,E)</em> and a source vertex <em>s</em>, how many BFS orderings are there and how would an algorithm look like to find them all?</p>
  
  <h2>Examples</h2>
  
  <p>A graph with only one vertex <em>A</em> has exactly one BFS ordering for <em>s=A</em>, namely [<em>A</em>].</p>
  
  <p>A graph with two connected vertices <em>A</em> and <em>B</em> has one BFS ordering for <em>s=A</em>, namely [<em>A</em>,<em>B</em>]</p>
  
  <p>A graph with tree vertices <em>A</em>,<em>B</em> and <em>C</em> all connected to each other has two BFS orderings for <em>s=A</em>, namely [<em>A</em>,<em>B</em>,<em>C</em>] and [<em>A</em>,<em>C</em>,<em>B</em>]</p>