Your university's board game club just hosted a Checkers tournament, and you were
assigned to take notes on the games. Unfortunately, while walking home, you dropped all
of your papers into a puddle! Disaster! Much of what you wrote is now unreadable; all you
have left are some lists of moves played in the middle of various games. Is there some
way you can reconstruct what happened in those games? You had better fix things fast, or
they will demote you to Tic-Tac-Toe recordkeeper! Checkers (or English Draughts) is a
well-known board game with simple rules. It is played on the dark squares of an 8 × 8
checkerboard. There are two players, Black and White, who alternate turns moving their
pieces (all of Black's pieces are black and all of White's pieces are white). Each piece
occupies a single dark square, and can be either a normal man or a promoted king. A turn
consists of choosing one piece and moving it in one of two ways:
1. Shifting it diagonally to an unoccupied adjacent dark square, as shown in Figure
1(a). This is called a simple move. If the piece is a man, it can move only in the two
diagonal directions towards the opposing side of the board (towards the bottom for
Black, the top for White). If the piece is a king, it can move in all four diagonal
directions.
2. Jumping over an adjacent enemy piece to an unoccupied square immediately on
the other side, then removing (capturing) that piece. Men can jump only in the two
directions described above, while kings can jump in all four. The player can then
repeat this step, continuing to jump with the same piece as long as there are
properly-positioned enemy pieces to capture. Such a sequence of one or more
jumps is called a jump move. Figure 1(b) shows a jump move comprising three
jumps.
In Checkers, captures are forced moves. If a jump move is available at the start of a
player's turn, they must jump, and cannot stop jumping with that piece until it has no more
possible jumps. They are free to choose which piece to jump with, and where, if there are
multiple possibilities. In Figure 1(b), Black could not have made any other move. If a man
reaches the farthest row from its player (that is, a black man reaches the bottom row or a
white man reaches the top row), it is removed from the board and replaced by a king of the
same color (it is said to be promoted), and the turn ends. A piece cannot be promoted and
then jump backwards as a new king in the same turn. Given a list of moves, find a setup
of pieces such that the moves can be legally played in sequence starting from that setup.
This setup may not have black men on the bottom row or white men on the top row, since
they would have been promoted to kings already. You need only ensure that the rules
above are obeyed; you do not need to ensure that this setup is reachable in a real game
of Checkers.