如何修改此代码以在选择下一个女王时考虑 MRV 策略？

如何解决如何修改此代码以在选择下一个女王时考虑 MRV 策略？

这是解决N-Queens问题的python源代码（假设n等于6）。

class NQueens:
    def __init__(self,size):
        # Store the puzzle (problem) size and the number of valid solutions
        self.size = size
        self.solutions = 0
        self.solve()

    def solve(self):
        positions = [-1] * self.size
        self.put_queen(positions,0)
        print("Found",self.solutions,"solutions.")
 
    def put_queen(self,positions,target_row):
        if target_row == self.size:
            self.show_full_board(positions)
            self.solutions += 1
        else:
            for column in range(self.size):
                if self.check_place(positions,target_row,column):
                    positions[target_row] = column
                    self.put_queen(positions,target_row + 1)

    def check_place(self,ocuppied_rows,column):
        for i in range(ocuppied_rows):
            if positions[i] == column or \
                positions[i] - i == column - ocuppied_rows or \
                positions[i] + i == column + ocuppied_rows:

                return False
        return True

    def show_full_board(self,positions):
        for row in range(self.size):
            line = ""
            for column in range(self.size):
                if positions[row] == column:
                    line += "Q "
                else:
                    line += ". "
            print(line)
        print("\n")

def main():
    NQueens(6)

if __name__ == "__main__":
    main()

什么是 MRV 策略？ 我如何修改此代码以在选择下一个女王时考虑 MRV 策略？