import java.io.*;
import java.util.*;

class C implements Serializable {
   public String id;
   public C l;
   public C p;

   public C(String id) {
       this.id = id;
   }

   public String s() {
      String s = id;

      if (l != null)
        s += " " + l.id;

      if (p != null)
        s += " " + p.id;

      return s;
   }

   public String r(List<C> list) {
      String s = id;
      list.add(this);

      if (l != null) {
        s += " " + l.id;

        if (!list.contains(l))
          s += " " + l.r(list);
      }

      if (p != null) {
        s += " " + p.id;

        if (!list.contains(l))
          s += " " + p.r(list);
      }

      return s;
   }

   public static void main(String[] args) throws ClassNotFoundException, IOException {

      C a = new C("a");
      C b = new C("b");
      C c = new C("c");
      C d = new C("d");
      C e = new C("e");

      a.l = c;
      a.p = d;
      b.l = d;
      c.l = b;
      e.l = b;

      ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("f.out"));
      out.writeObject(e);
      out.close();

      ObjectInputStream in = new ObjectInputStream(new FileInputStream("f.out"));
      C x = (C) in.readObject();
      in.close();

      System.out.println(x.r(new ArrayList<C>()));
   }
}