https://leetcode.com/problems/sudoku-solver

sudoku_solver.py

class Solution:
    VALID_VALUES = set(map(str, range(1, 10)))

    def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        def rows(row):
            return board[row]

        def columns(column):
            for row in range(len(board)):
                yield board[row][column]

        def squares(row, column):
            start_row = (row // 3) * 3
            start_column = (column // 3) * 3
            for row in range(start_row, start_row+3):
                for column in range(start_column, start_column+3):
                    yield board[row][column]

        def is_set(value):
            return value != '.'

        def safe(row, column):
            return self.VALID_VALUES - (set(rows(row)) | set(columns(column)) | set(squares(row, column)))

        def unset():
            for row in range(len(board)):
                for column in range(len(board[0])):
                    if not is_set(board[row][column]):
                        yield row, column

        def solve(pos_idx, positions):
            row, column = positions[pos_idx]
            for safe_value in safe(row, column):
                board[row][column] = safe_value
                if pos_idx == len(positions) - 1:
                    return True
                solution = solve(pos_idx+1, positions)
                if solution:
                    return True
            board[row][column] = '.'
        
        unset = list(unset())
        if not unset:
            return  # already solved
        
        solve(0, unset)