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Who played first? (Posted on 2004-05-07) Difficulty: 3 of 5
Given this tic-tac-toe position, as played by two expert players, who went first and where? Also, which was the last "move"?
 X | O | O 
---+---+---
   |   | X
---+---+---
   | X | O
(An expert player is a player who would never play in such a way that would allow his opponent to win, and who would also try to get the possible best result.)

See The Solution Submitted by e.g.    
Rating: 3.7000 (10 votes)

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Hints/Tips first step or two | Comment 3 of 26 |

I haven't read the other comments yet.

Two tic-tac-toe experts will always tie.  So, these two experts will tie, and the only limit to their possible moves is that they won't let the other player to force a win.

If the first player plays on a corner, the other player must play on the center, or else the first player will be able to force a win by making one of the following moves (X represents the first move, x the third):

X|O|   X| |O  X| |   X| |x
 |x|    | |    | |O   | | 
 | |   x| |   x| |    | |O

So the first move cannot be in a corner, but must be on a side.

The second move must be next to or accross from the first move, or a forced win is possible again:

x|X|   x|X| 
O| |    | | 
 | |   O| |

This still leaves 6 different possibilities for the first two moves, so now I'm stuck.


  Posted by Tristan on 2004-05-07 17:25:25
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