Earlier today I present you a puzzle about a variant of zeros and crosses (tic-tac-toe in the US). Games are played exactly as normal with the following twist:
Players can place an X or an O each time.
That’s right, you can swing both ways. A player can play X at one time and O at the next, or vice versa, or stick with the same symbol. On each path it is up to the player to decide. As usual, the first player to get a line of three O’s or X’s wins.
Today’s puzzle asked: if you go first, how do you always guarantee a win?
Before you browse to the solution, you may want to check out the original postwhich had a sample game.
Solution
Player 1 (red) goes first in the center with eg X. This forces player 2 to a ohas a 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 Riddles, Perplexing Paradoxes, Confounding 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 doing a puzzle here on alternate Mondays since 2015. I’m always on the lookout for great puzzles. If you want 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 5 September. To support the Guardian and Observer, order your copy from guardianbookshop.com. Delivery charges may apply.
