Question

I am fairly new to java and I have been struggling with this exercise for two weeks now(It’s an homework exercise in my school). I need to create a topological sort and print out all of the possible connections. I have read a lot about topological sorting now, but we have this certain line of code that we have to work with. I’m pretty sure I could do the topological sorting when I have the list of vertices. My problem is, I don’t know how to list all of the vertices from this given code. Could anyone give me some tips or leads or perhaps an example, I would really really appreciate it.

Here is the given code we need to work with:

import java.util.*;

public class Answer {

   public static void main (String[] args) {
      Answer a = new Answer();
      a.run();
   }

   public void run() {

      // TODO!!! YOUR TESTS HERE!

      Graph g = new Graph ("G");
      Vertex a = new Vertex ("A");
      Vertex b = new Vertex ("B");
      Vertex c = new Vertex ("C");
      g.first = a;
      a.next = b;
      b.next = c;
      Edge ab = new Edge ("AB");
      Edge ac = new Edge ("AC");
      Edge ba = new Edge ("BA");
      Edge ca = new Edge ("CA");
      a.first = ab;
      b.first = ba;
      c.first = ca;
      ab.next = ac;
      ab.target = b;
      ac.target = c;
      ba.target = a;
      ca.target = a;
      System.out.println (g);
   }


   class Vertex {

      String id;
      Vertex next;
      Edge first;

      Vertex (String s, Vertex v, Edge e) {
         id = s;
         next = v;
         first = e;
      } 

      Vertex (String s) {
         this (s, null, null);
      }

      @Override
      public String toString() {
         return id;
      }

      // TODO!!! Your Vertex methods here!

   } // Vertex


   class Edge {

      String id;
      Vertex target;
      Edge next;

      Edge (String s, Vertex v, Edge e) {
         id = s;
         target = v;
         next = e;
      }

      Edge (String s) {
         this (s, null, null);
      }

      @Override
      public String toString() {
         return id;
      }

      // TODO!!! Your Edge methods here!

   } // Edge


   class Graph {

      String id;
      Vertex first;

      Graph (String s, Vertex v) {
         id = s;
         first = v;
      }

      Graph (String s) {
         this (s, null);
      }

      @Override
      public String toString() {
         String nl = System.getProperty ("line.separator");
         StringBuffer sb = new StringBuffer (nl);
         sb.append (id + nl);
         Vertex v = first;
         while (v != null) {
            sb.append (v.toString() + " --> ");
            Edge e = v.first;
            while (e != null) {
               sb.append (e.toString());
               sb.append ("(" + v.toString() + "->" 
                  + e.target.toString() + ") ");
               e = e.next;
            }
            sb.append (nl);
            v = v.next;
         }
         return sb.toString();
      }

      // TODO!!! Your Graph methods here!

   } // Graph
}

Answer 1

Apparently, the graph has a reference to the first vertex, and the vertices themselves are linked together into a singly linked list. This code should be all you need to collect the vertices into a Java list:

public List<Vertex> allVertices(Graph g) {    
  final List<Vertex> vertices = new ArrayList<>();
  for (Vertex v = g.first; v != null; v = v.next)
     vertices.add(v);
  return vertices;
}