Tic Tac Toe

Despite its apparent simplicity, Tic-tac-toe requires detailed analysis to determine even some elementary combinatory facts, the most interesting of which are the number of possible games and the number of possible positions. A position is merely a state of the board, while a game usually refers to the way a terminal position is obtained.

The game ends when three-in-a-row is obtained.
The number of Xs is always either equal to or exactly 1 more than the number of Os (if X starts).