def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        #https://leetcode.com/discuss/84831/java-backtracking-stack-20ms
        empty = []
        for i in range(9):
            for j in range(9):
                if board[i][j] == '.':
                    empty.append(9 * i + j)
        self.solve(board, empty)

        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        
python/037_Sudoku_Solver.py