I just used the coordiantes of the grid to check all possible winning combinations.

ttt/game.py

@@ -58,4 +58,14 @@ class TTTGame:
 
     def check_winner(self, state, symbol):
         "Checks whether the player with `symbol` has won the game."
+        board = state["board"]
+        # All winning triplets (rows, columns, diagonals)
+        winning_combinations = [
+            (0, 1, 2), (3, 4, 5), (6, 7, 8),  # rows
+            (0, 3, 6), (1, 4, 7), (2, 5, 8),  # columns
+            (0, 4, 8), (2, 4, 6),             # diagonals
+        ]
+        for i, j, k in winning_combinations:
+            if board[i] == board[j] == board[k] == symbol:
+                return True
         return False