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’s or X’s?
If it is X’s turn it would mean O’s failed to block X’s open threat. Since we know neither player would make such an error, it must therefore be O’s turn in the middle cell of the left column – Figure 1.
X’s then play in the upper right corner, leading to two winning moves indicated by grey X’s – Figure 2. O’s can not block both cells simultaneously, so X’s win.