Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

1. Each of the digits 1-9 must occur exactly once in each row.
2. Each of the digits 1-9 must occur exactly once in each column.
3. Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.

The '.' character indicates empty cells.

Example 1:

Input: board = [["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]
Output: [["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
Explanation: The input board is shown above and the only valid solution is shown below:

Constraints:

• board.length == 9
• board[i].length == 9
• board[i][j] is a digit or '.'.
• It is guaranteed that the input board has only one solution.

SOLUTION:

class Solution:
    def getPos(self, board, i, j):
        pos = set(map(str, range(1, 10)))
        ci, cj = i - i % 3, j - j % 3
        for a in range(3):
            for b in range(3):
                if board[ci + a][cj + b] != "." and board[ci + a][cj + b] in pos:
                    pos.remove(board[ci + a][cj + b])
        for a in range(9):
            if board[i][a] in pos:
                pos.remove(board[i][a])
            if board[a][j] in pos:
                pos.remove(board[a][j])
        return pos

    def solve(self, board, spi):
        if spi >= len(self.spaces):
            return True
        if spi < len(self.spaces):
            i, j = self.spaces[spi]
            pos = self.getPos(board, i, j)
            for p in pos:
                board[i][j] = p
                currsolution = self.solve(board, spi + 1)
                if currsolution:
                    return True
                else:
                    board[i][j] = "."
        return False

    def solveSudoku(self, board: List[List[str]]) -> None:
        self.spaces = []
        for i in range(9):
            for j in range(9):
                if board[i][j] == ".":
                    self.spaces.append((i, j))
        self.solve(board, 0)