IMO Shortlist 1994 problem C6

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Dodao/la: arhiva
2. travnja 2012.
Two players play alternatively on an infinite square grid. The first player puts a X in an empty cell and the second player puts a O in an empty cell. The first player wins if he gets 11 adjacent Xs in a line, horizontally, vertically or diagonally. Show that the second player can always prevent the first player from winning.
Izvor: Međunarodna matematička olimpijada, shortlist 1994