Earlier today I set you a puzzle about a variant of noughts and crosses (tic-tac-toe in the US). Games are played exactly as normal with the following twist:
Players can place either an X or an O on each go.
That’s right, you can swing both ways. A player can play X on one go and O on the next, or vice versa, or stick with the same symbol. On each go, it is up to the player to decide. As per usual, the first player to get a line of three Os or Xs wins.
Today’s puzzle asked: if you go first, how do you always guarantee a win?
Before you scroll to the solution, you might want to check out the original post, which had an example game.
Solution
Player 1 (red) goes first in the middle with, say X. This forces player 2 to place an O, as an X placed anywhere will let player 1 win.
Scenario 1: Player 2 places an O in the corner
Scenario 2: Player 2 places an O on the side.
Thanks to John Driscoll, the author of A Hen and a Half, Intriguing Conundrums, Confusing Paradoxes, Baffling Conjectures and Challenging Puzzles, where I first read about this puzzle. Readers of this column will find his book full of entertaining material.
I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.
My new book, Think Twice: Solve the simple puzzles (almost) everyone gets wrong (Square Peg, £12.99), is out on September 5. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.
