Sudoku Solver

sudoku_solver.py

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):
        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)
        
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"],
]

Solution().solveSudoku(board)

print("\n".join([" ".join(row) for row in board]))

# 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