今有 6 x 6 的棋盘格。其中某些格子已经预先放好了棋子。现在要再放上去一些，使得：每行每列都正好有3颗棋子。我们希望推算出所有可能的放法。下面的代码就实现了这个功能。-This checkerboard of 6 x 6. Some lattice has been pre-put away a pawn. Recapture up, such that: each row and each column exactly three pieces. We want to figure out