Traverse.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class Traverse {
  public static void main(String[] args) {
    // Create Person objects with empty confidant sets.
    Person alice   = new Person("Alice",   30, new HashSet<>());
    Person bob     = new Person("Bob",     28, new HashSet<>());
    Person charlie = new Person("Charlie", 32, new HashSet<>());
    Person diana   = new Person("Diana",   27, new HashSet<>());
    Person eve     = new Person("Eve",     35, new HashSet<>());
    Person frank   = new Person("Frank",   29, new HashSet<>());
    Person grace   = new Person("Grace",   33, new HashSet<>());
    Person henry   = new Person("Henry",   31, new HashSet<>());
    Person irene   = new Person("Irene",   26, new HashSet<>());
    Person jack    = new Person("Jack",    40, new HashSet<>());

    // Set up confidant relationships.
    // Note: Relationships are directional and a person is not their own confidant.
    alice.getConfidants().addAll(Arrays.asList(bob, charlie, diana, frank));
    bob.getConfidants().addAll(Arrays.asList(alice, eve));
    charlie.getConfidants().addAll(Arrays.asList(bob, frank));
    diana.getConfidants().addAll(Arrays.asList(charlie, grace, eve));
    eve.getConfidants().addAll(Arrays.asList(frank, irene));
    frank.getConfidants().addAll(Arrays.asList(bob, grace));
    grace.getConfidants().add(henry);
    henry.getConfidants().addAll(Arrays.asList(irene));
    irene.getConfidants().addAll(Arrays.asList(jack));
    jack.getConfidants().addAll(Arrays.asList(henry));


    Map<Integer, Set<Integer>> graph = new HashMap<>();
    graph.put(3, new HashSet<>(Arrays.asList(7, 34)));
    graph.put(7, new HashSet<>(Arrays.asList(12, 45, 34, 56)));
    graph.put(12, new HashSet<>(Arrays.asList(7, 56, 78)));
    graph.put(34, new HashSet<>(Arrays.asList(34, 91)));
    graph.put(56, new HashSet<>(Arrays.asList(78)));
    graph.put(78, new HashSet<>(Arrays.asList(91)));
    graph.put(91, new HashSet<>(Arrays.asList(56)));
    graph.put(45, new HashSet<>(Arrays.asList(23)));
    graph.put(23, new HashSet<>());
    graph.put(67, new HashSet<>(Arrays.asList(91)));

    // See below site for visualization of this graph
    // https://auberonedu.github.io/graph-explore/graph_site/viz.html
    Vertex<Integer> v3  = new Vertex<>(3);
    Vertex<Integer> v7  = new Vertex<>(7);
    Vertex<Integer> v12 = new Vertex<>(12);
    Vertex<Integer> v34 = new Vertex<>(34);
    Vertex<Integer> v56 = new Vertex<>(56);
    Vertex<Integer> v78 = new Vertex<>(78);
    Vertex<Integer> v91 = new Vertex<>(91);
    Vertex<Integer> v45 = new Vertex<>(45);
    Vertex<Integer> v23 = new Vertex<>(23);
    Vertex<Integer> v67 = new Vertex<>(67);

    v3.neighbors  = new ArrayList<>(List.of(v7, v34));
    v7.neighbors  = new ArrayList<>(List.of(v12, v45, v34, v56));
    v12.neighbors = new ArrayList<>(List.of(v7, v56, v78));
    v34.neighbors = new ArrayList<>(List.of(v34, v91)); 
    v56.neighbors = new ArrayList<>(List.of(v78));
    v78.neighbors = new ArrayList<>(List.of(v91));
    v91.neighbors = new ArrayList<>(List.of(v56));
    v45.neighbors = new ArrayList<>(List.of(v23));
    v23.neighbors = new ArrayList<>(List.of());
    v67.neighbors = new ArrayList<>(List.of(v91));

    System.out.println(reachable(graph, 3, 45));
  }

  //HashMap traversal
  public static boolean reachable(Map<Integer,Set<Integer>> graph, int start, int end){
    Set<Integer> seen = new HashSet<>();
    return reachable(graph, start, end, seen);
  }

  private static boolean reachable(Map<Integer,Set<Integer>> graph, int start, int end, Set<Integer> seen){
    if(start == end) return true;
    if(seen.contains(start) || graph == null || !graph.containsKey(start) || graph.get(start) == null) return false;
    seen.add(start);

    for (int neighbor : graph.get(start)) {
      if(reachable(graph, neighbor, end, seen)) return true;
    }

    return false;
  }

  //Person traversal
  public static void printGossipers(Person person){
    printGossipers(person, new HashSet<>());
  }

  private static void printGossipers(Person person, Set<Person> seen){
    if(person == null || seen.contains(person)) return;
    seen.add(person);
    System.err.println(person.getName());

    for (Person confidant : person.getConfidants()) {
      printGossipers(confidant, seen);
    }
  }
}