// Textbook fragment 13.04
/** Generic DFS traversal of a graph using the template method pattern.
  * A subclass should override various methods to add functionality.
  * Parameterized types:
  *   V, the type for the elements stored at vertices
  *   E, the type for the elements stored at edges
  *   I, the type for the information object passed to the execute method
  *   R, the type for the result object returned by the DFS
  */
public class DFS<V, E, I, R> {
  protected Graph<V, E> graph;    // The graph being traversed
  protected Vertex<V> start;      // The start vertex for the DFS
  protected I info;               // Information object passed to DFS
  protected R visitResult;        // The result of a recursive traversal call
  protected static Object STATUS = new Object();    // The status attribute
  protected static Object VISITED = new Object();   // Visited value
  protected static Object UNVISITED = new Object(); // Unvisited value