Crisscross 2023

Crisscross is a two-player game played on a square board of any size, initially empty. The two players, Red and Blue, take turns placing their own checkers onto unoccupied squares on the board, one checker per turn, starting with Red. Passing is not allowed.

Red must form a path of red checkers (interconnected via horizontal or vertical adjacencies, or both) connecting the two red sides of the board. Diagonal adjacencies are irrelevant in Crisscross. Blue must form a path of blue checkers connecting the two blue sides of the board.

It’s unknown if a deadlock can occur. If it is possible, it’s extremely unlikely.

SegmentA segment is a straight (horizontal or vertical), contiguous sequence of like colored checkers, of length one or more.

When you place a checker, you form a new segment. Your newly placed checker will either form a segment of length one, or it will lengthen an existing segment (or two perpendicular segments), or it will combine two existing segments into a larger segment, or some combination thereof.

Flanking and capture (different from Reversi)A blue segment is flanked if it is sandwiched between two red segments in a straight line. Conversely for a flanked red segment.

If your new segment, together with another of your segments, flanks an enemy segment, replace all checkers of the enemy segment with your own checkers, concluding your turn.

PerimeterConsider the board to be surrounded by an invisible perimeter of squares. You can never place a checker on this imaginary perimeter. But, on your turn, assume that the perimeter is entirely occupied by your checkers.