Tic-Tac-Toe

Tic-Tac-Toe is a simple game played on a 3x3 grid between two people. Players take turns placing either an X or an O into one of the nine spaces. A player wins by getting three of their marks in a horizontal, vertical or diagonal row.

Example Tic-Tac-Toe Game where X goes first and wins.

If you exclude rotation and reflection, there are only 765 different states for the game to be in, and only 138 final outcomes for the game. If X goes first, there are 91 winning outcomes, 44 losing outcomes and 3 draws (ties).

If both players play perfectly every game will end in a draw. In order to play perfectly, you must follow this simple strategy.

Strategy

To play perfectly, you should make the first possible move from the following ordered list.

1. Win - If you can get three in a row, do it.
2. Block a Win - If your opponent has two in a row, block them.
3. Fork - Place your mark in such a way that you have two ways to win on your next move.
4. Block a Fork - If your opponent's next move provides a way for them to fork, block this, preferably by getting your own two-in-a-row.
5. Play the Center - If the center is empty, take it.
6. Play a Corner - If a corner is empty, take it, preferably opposite the corner the opponent is occupying.
7. Play a Side - If a side square is empty, take it.