Added utilities and a hashmap inverter.

src/main/java/ca/recrown/islandsurvivalcraft/Utilities.java

@@ -0,0 +1,21 @@
+package ca.recrown.islandsurvivalcraft;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map.Entry;
+
+public class Utilities {
+    public static <K, V> HashMap<V, ArrayList<K>> invertHashMap(HashMap<K, V> hashMap) {
+        HashMap<V, ArrayList<K>> res = new HashMap<>();
+        for (Entry<K, V> entry : hashMap.entrySet()) {
+            if (!res.containsKey(entry.getValue())) {
+                ArrayList<K> l = new ArrayList<K>();
+                l.add(entry.getKey());
+                res.put(entry.getValue(), l);
+            } else {
+                res.get(entry.getValue()).add(entry.getKey());
+            }
+        }
+        return res;
+    }
+}