Rules of the Game

In case there is anyone who has never played this game, TicTacToe (also called Noughts and Crosses, Xs and Os) is a pencil-and-paper game for two players, X and O, who take turns marking the spaces in a 3×3 grid. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game.

The following example game is won by the first player, X:

TicTacToe Game

Players soon discover that best play from both parties leads to a draw (often referred to as cat or cat's game). Hence, Tic-tac-toe is most often played by young children.

Strategy

A player can play perfect TicTacToe (win or draw) given they choose the first possible move from the following list.

• Win: If the player has two in a row, they can place a third to get three in a row.
• Block: If the [opponent] has two in a row, the player must play the third themself to block the opponent.
• Fork: Creation of an opportunity where the player has two threats to win (two non-blocked lines of 2).
• Blocking an opponent's fork:
  • Option 1: The player should create two in a row to force the opponent into defending, as long as it doesn't result in them creating a fork. For example, if "X" has a corner, "O" has the center, and "X" has the opposite corner as well, "O" must not play a corner in order to win. (Playing a corner in this scenario creates a fork for "X" to win.)
  • Option 2: If there is a configuration where the opponent can fork, the player should block that fork.
• Center: A player marks the center. (If it is the first move of the game, playing on a corner gives "O" more opportunities to make a mistake and may therefore be the better choice; however, it makes no difference between perfect players.)
• Opposite corner: If the opponent is in the corner, the player plays the opposite corner.
• Empty corner: The player plays in a corner square.
• Empty side: The player plays in a middle square on any of the 4 sides.