interview-preparation-kit/Leetcode/python/37-sudoku-solver.py at master · maze-runnar/interview-preparation-kit · GitHub

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"""
37. Sudoku Solver
Hard - 65.3%

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"]]
"""

from typing import List

class Solution:
    def solveSudoku(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        self.solve(board)

    def solve(self, board: List[List[str]]) -> bool:
        for i in range(9):
            for j in range(9):
                if board[i][j] == '.':
                    for c in '123456789':
                        if self.is_valid(board, i, j, c):
                            board[i][j] = c

                            if self.solve(board):
                                return True

                            board[i][j] = '.'  # backtrack

                    return False
        return True

    def is_valid(self, board: List[List[str]], row: int, col: int, c: str) -> bool:
        for i in range(9):
            # Check row
            if board[row][i] == c:
                return False

            # Check column
            if board[i][col] == c:
                return False

            # Check 3x3 box
            if board[3 * (row // 3) + i // 3][3 * (col // 3) + i % 3] == c:
                return False

        return True