Checkpoint 2: List all the possible cases for winning first.

Then I thought of for loop, and realized that the work will be much longer than I wished while making a note on the paper.
Index would be more ideal to check each combination with a shorter code.

ttt/game.py

@@ -58,4 +58,9 @@ class TTTGame:
 
     def check_winner(self, state, symbol):
         "Checks whether the player with `symbol` has won the game."
+        winning_combinations=[[0,1,2],[3,4,5],[6,7,8],[0,3,6],[1,4,7],[2,5,8],[0,4,8],[2,4,6]]
+        board=state["board"]
+        combination=[index for index in range(9) if board[index]==symbol]
+        if combination in winning_combinations:
+            return True
         return False