Add day 14

EnumerableExtensions.cs

@@ -10,11 +10,16 @@ public static class EnumerableExtensions
         while (toVisit.TryPop(out var node))
         {
             yield return node;
-        
+
             foreach (var child in flattener(node))
             {
                 toVisit.Push(child);
             }
         }
     }
+
+    public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
+    {
+        return dictionary.TryGetValue(key, out var value) ? value : defaultValue;
+    }
 }