Fixed TTTGame.check_winner.

What I changed
Added 8 cases to check for to see if someone won: three rows, three columns and two diagonals.

Why I changed it
TTTGame.check_winner always reported false, so the game only ended when the board was full even if someone should have won.
Estimate for remaining time to finish assignment: [Like a few hours assuming my students don't rip a door off its hinges in the bathroom again...]

notes.md

@@ -30,6 +30,8 @@ and it's your turn, which action would you take? Why?
 ---+---+---   ---+---+---   ---+---+---   ---+---+---
    |   |         |   | O       |   |         |   |   
 
+For the first one, I would put an X on the rightmost cell in the middle row. This would allow me to get a win. For the second one, I would put it in the same position as I would have in the first. This prevents O from winning on the next turn and puts me one turn away from winning, assuming the opponent doesn't block that route. For the third one, I would put it in the top left. This gives me two possible ways to win, and since the opponent only has one turn after mine before I get to go again, they cannot stop me from winning. For the last one, I would put it in the center. The opponent has to block by placing an O in the bottom right, and then I can put an X in the leftmost cell of the middle row. This leads to two possible ways to win again.
+
 ### Initial game state
 
 You can get the inital game state using game.get_initial_state(). 

ttt/game.py

@@ -58,4 +58,20 @@ class TTTGame:
 
     def check_winner(self, state, symbol):
         "Checks whether the player with `symbol` has won the game."
+        if state["board"][0] == symbol and state["board"][1] == symbol and state["board"][2] == symbol:
+            return True
+        elif state["board"][3] == symbol and state["board"][4] == symbol and state["board"][5] == symbol:
+            return True
+        elif state["board"][6] == symbol and state["board"][7] == symbol and state["board"][8] == symbol:
+            return True
+        elif state["board"][0] == symbol and state["board"][3] == symbol and state["board"][6] == symbol:
+            return True
+        elif state["board"][1] == symbol and state["board"][4] == symbol and state["board"][7] == symbol:
+            return True
+        elif state["board"][2] == symbol and state["board"][5] == symbol and state["board"][8] == symbol:
+            return True
+        elif state["board"][0] == symbol and state["board"][4] == symbol and state["board"][8] == symbol:
+            return True
+        elif state["board"][2] == symbol and state["board"][4] == symbol and state["board"][6] == symbol:
+            return True
         return False