We create an AI based on Minimax. The computer cannot know what his opponent will do in the next steps, but he can check what he can do in the worst-case. The minimum outcome of these worst cases is maximized by this algorithm. This behavior has given Minimax its name.
To learn how Minimax works, we will take a look at the value or score of a grid. If the game is finished, we can easily define its value: if you won, the value is 1; if you lost, -1 and if it is a draw, 0. Thus, for player 1 the next grid has value 1 and for player 2 the value is -1:
X|X|X -+-+- O|O| -+-+- X|O|
We will also define the value of a grid for a game that has not been finished. We take a look at the following grid:
X| |X -+-+- O|O|