Puzzle #2: Reverse Tic-Tac-Toe
The tic-tac-toe position below was reached in a game between two flawless players, meaning neither of them ever made a move that would allow their opponent to win by force and neither of them missed an opportunity to force a win themselves. What were the last four moves played?
You may already know that, with perfect play, tic-tac-toe always ends in a tie where neither player achieves three-in-a-row. So it’s no surprise that a game between two flawless players looks to be headed for stalemate. But how did we get here? To uncover the past, you’ll really need to convince yourself of each move. Producing a plausible-looking sequence of moves will not suffice, because only one such sequence could have occurred under the stipulation of flawless play. Although tic-tac-toe is a small game, the solution requires a sizable dose of deduction.
Find the solution at the bottom of this page.
