title: BFS tree to graph
body: <p>Suppose we obtain the following BFS tree rooted at node D for an undirected graph with vertices {A,B,C,D,E,F,G,H}.
  <img src="https://i.stack.imgur.com/cicv0.png" alt="Tree" />
  How to determine whether a particular edge is present or not in the original graph?</p>
  <p>This is a multiple choice type question:</p>
  <blockquote>
  <p>Which of the following edges is not present in the original graph?</p>
  <ol>
  <li>(F, G)</li>
  <li>(B, E)</li>
  <li>(A, G)</li>
  <li>(E, H)</li>
  </ol>
  </blockquote>