by Serhiy Grabarchuk Jr
The diagram shows a tic-tac-toe game somewhere in the process. It is known the game ended with a win.
Assuming that both players are equally skilled and do not miss a chance to directly block the opponent from completing a 3-in-a-row line, who eventually won this game – O or X?
If it would be O’s turn, O’s would win right away by merely placing an O in the upper right corner. That in turn would mean X’s didn’t block the O’s open threat at their previous move. Since we know neither player would make such an error, it must be X’s turn now.
X’s will play in the upper right corner to block the O’s winning move. X’s can then win by playing in either cell indicated by a grey X, since O’s cannot block both cells with their respective move.