Add solution for 2016 Day 1

src/common/lib/Array2D.h

@@ -9,6 +9,20 @@ struct Index2D
   int x = -1;
   int y = -1;
 
+  void RotateCW()
+  {
+    int tmp = y;
+    y = x;
+    x = -tmp;
+  }
+
+  void RotateCCW()
+  {
+    int tmp = y;
+    y = -x;
+    x = tmp;
+  }
+
   bool IsValid()
   {
     return x != -1 && y != -1;