unified code for cumputing next player

lib/util.dart

@@ -25,6 +25,18 @@ class Util {
         emptyBoardClassic,
       ];
 
+  static TTCState nextTurn(TTCState currentPlayer,
+      {TTCState defaultState = TTCState.x}) {
+    switch (currentPlayer) {
+      case TTCState.x:
+        return TTCState.o;
+      case TTCState.o:
+        return TTCState.x;
+      default:
+        return defaultState;
+    }
+  }
+
   static String stateText(TTCState state) => state.name.toUpperCase();
   static String cellAddress(int index) =>
       "${index % 3}, ${(index / 3).floor()}";