Validating a Sudoku Board in Python

 

Validating a Sudoku Board in Python 

Sudoku is a classic puzzle that challenges players to fill a 9×9 grid so that each row, column, and 3×3 subgrid contains all digits from 1 to 9 without repetition. While solving it is fun, validating a given board programmatically is a great exercise in logic and Python skills.

In this post, I’ll walk you through how I built a Python function to validate a Sudoku board. The goal is to check whether the board follows the rules of Sudoku—even if it's not fully solved.

🔍 Problem Statement

Given a 9×9 board filled with digits '1' to '9' and empty cells represented by '.', determine if the board is valid. A valid board must satisfy:

• Each row contains unique digits.

• Each column contains unique digits.

• Each 3×3 subgrid contains unique digits.

🧠 My Solution Strategy

I broke the problem into three parts:

1. Validate each row.

2. Validate each column.

3. Validate each 3×3 subgrid.

Here’s the complete code:

from typing import List

class Solution:

    def get_submatrix(self, sudoku):

        subgrids = []

        for row_block in range(3):

            for col_block in range(3):

                start_row = row_block * 3

                start_col = col_block * 3

                subgrid = [row[start_col:start_col+3] for row in sudoku[start_row:start_row+3]]

                subgrids.append(subgrid)

        return subgrids

    def isValidSudoku(self, board: List[List[str]]) -> bool:

        row = 0

        validRow = 1

        validColumn = 1

        validMartrix = 1

        # Validating rows

        for val in board:

            row += 1

            row_data = []

            for i, data in enumerate(val):

                if data in row_data and data != ".":

                    validRow = 0

                row_data.append(data)

        # Validating columns

        inverseBoard = [list(row) for row in zip(*board)]

        for val in inverseBoard:

            row += 1

            row_data = []

            for i, data in enumerate(val):

                if data in row_data and data != ".":

                    validColumn = 0

                row_data.append(data)

        # Print each 3x3 subgrid

        subgrids = self.get_submatrix(board)

       

       

        for i, grid in enumerate(subgrids):

            row_data = []

            for row in grid:              

                for subelement in row:                  

                    if subelement in row_data and subelement != ".":

                      validMartrix = 0

                    row_data.append(subelement)

               

        if(validRow==0 or validColumn==0 or validMartrix==0):

           print("valid",validMartrix)

           return False

        else:

            return True

# Sample board

board = [["5", "3", ".", ".", "7", "8", ".", ".", "."],

         ["6", ".", ".", "1", "9", "5", ".", ".", "."],

         ["5", "9", "8", ".", ".", ".", ".", "6", "."],

         ["8", ".", ".", ".", "6", ".", ".", ".", "3"],

         ["4", ".", ".", "8", ".", "3", ".", ".", "1"],

         ["7", ".", ".", ".", "2", ".", ".", ".", "6"],

         [".", "6", ".", ".", ".", ".", "2", "8", "."],

         [".", ".", ".", "4", "1", "9", ".", ".", "5"],

         [".", ".", ".", ".", "8", ".", ".", "7", "9"]]

# Run validation

obj = Solution()

obj.isValidSudoku(board)