Completed all methods, and passed all tests.

src/Build.java

@@ -4,7 +4,8 @@
 import java.util.Set;
 
 
-public class Build {
+public class Build 
+{
 
   /**
    * Prints words that are reachable from the given vertex and are strictly shorter than k characters.
@@ -13,7 +14,19 @@ public class Build {
    * @param vertex the starting vertex
    * @param k the maximum word length (exclusive)
    */
-  public static void printShortWords(Vertex<String> vertex, int k) {
+  public static void printShortWords(Vertex<String> vertex, int k) 
+  {
+    Set<Vertex<String>> visited = new HashSet<>();
+    printShortWords(vertex, k, visited);
+
+  }
+  public static void printShortWords(Vertex<String> vertex, int k, Set<Vertex<String>> visited)
+  {
+    if (vertex == null || visited.contains(vertex)) return;
+    visited.add(vertex);
+    if (vertex.data.length() < k) System.out.println(vertex.data);
+    for (Vertex<String> neighbor : vertex.neighbors) printShortWords(neighbor, k, visited);
+
   }
 
   /**
@@ -22,8 +35,19 @@ public static void printShortWords(Vertex<String> vertex, int k) {
    * @param vertex the starting vertex
    * @return the longest reachable word, or an empty string if the vertex is null
    */
-  public static String longestWord(Vertex<String> vertex) {
-    return "";
+  public static String longestWord(Vertex<String> vertex) 
+  {
+    Set<Vertex<String>> visited = new HashSet<>();
+    String longest = "";
+    return longestWord(vertex, visited, longest);
+  }
+  public static String longestWord(Vertex<String> vertex, Set<Vertex<String>> visited, String longest)
+  {
+    if (vertex == null || visited.contains(vertex)) return longest;
+    visited.add(vertex);
+    if (vertex.data.length() > longest.length()) longest = vertex.data;
+    for (Vertex<String> neighbor : vertex.neighbors) return longestWord(neighbor, visited, longest);
+    return longest;
   }
 
   /**
@@ -33,7 +57,19 @@ public static String longestWord(Vertex<String> vertex) {
    * @param vertex the starting vertex
    * @param <T> the type of values stored in the vertices
    */
-  public static <T> void printSelfLoopers(Vertex<T> vertex) {
+  public static <T> void printSelfLoopers(Vertex<T> vertex) 
+  {
+    Set<Vertex<T>> visited = new HashSet<>();
+    printSelfLoopers(vertex, visited);
+
+  }
+  public static <T> void printSelfLoopers(Vertex<T> vertex, Set<Vertex<T>> visited)
+  {
+    if (vertex == null || visited.contains(vertex)) return;
+    visited.add(vertex);
+    if (vertex.neighbors.contains(vertex)) System.out.println(vertex.data);
+    for (Vertex<T> neighbor : vertex.neighbors) printSelfLoopers(neighbor, visited);
+
   }
 
   /**
@@ -44,8 +80,22 @@ public static <T> void printSelfLoopers(Vertex<T> vertex) {
    * @param destination the destination airport
    * @return true if the destination is reachable from the start, false otherwise
    */
-  public static boolean canReach(Airport start, Airport destination) {
+  public static boolean canReach(Airport start, Airport destination)
+  {
+    Set<Airport> visited = new HashSet<>();
+    return canReach(start, destination, visited);
+  }
+  public static boolean canReach(Airport start, Airport destination, Set<Airport> visited)
+  {
+    if (start == destination) return true;
+    if (start == null || destination == null || visited.contains(start)) return false;
+    visited.add(start);
+    for (Airport flight : start.getOutboundFlights())
+    {
+      if (canReach(flight, destination, visited)) return true;
+    }
     return false;
+
   }
 
   /**
@@ -57,7 +107,20 @@ public static boolean canReach(Airport start, Airport destination) {
    * @param <T> the type of values stored in the graph
    * @return a set of values that cannot be reached from the starting value
    */
-  public static <T> Set<T> unreachable(Map<T, List<T>> graph, T starting) {
-    return new HashSet<>();
+  public static <T> Set<T> unreachable(Map<T, List<T>> graph, T starting)
+  {
+    if (graph == null) return new HashSet<>();
+    Set<T> unvisited = new HashSet<>();
+    for (T key : graph.keySet()) unvisited.add(key);
+    if (!graph.containsKey(starting)) return unvisited;
+    return unreachable(graph, starting, unvisited);
   }
+  public static <T> Set<T> unreachable(Map<T, List<T>> graph, T starting, Set<T> unvisited)
+  {
+    if (starting == null || !unvisited.contains(starting)) return unvisited;
+    unvisited.remove(starting);
+    for (T nextPoint : graph.get(starting)) unreachable(graph, nextPoint, unvisited);
+    return unvisited;
+  }
+
 }