Sudoku solver

class Solution:
    def backtrack(self, board):
        for i in range(len(board)):
            for j in range(len(board[0])):
                if board[i][j] != '.':
                    continue
                for k in range(1, 10):
                    if self.chackValid(board, i, j, str(k)):
                        board[i][j] = str(k)
                        if self.backtrack(board): return True
                        board[i][j] = "."
                return False
        return True

    def chackValid(self, board, row, col, k):
        for i in range(9):
            if board[row][i] == k:
                return False
        for i in range(9):
            if board[i][col] == k:
                return False
        startRow = (row // 3) * 3
        startCol = (col // 3) * 3

        for i in range(startRow, startRow + 3):
            for j in range(startCol, startCol + 3):
                if board[i][j] == k:
                    return False
        return True

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

Sudoku Solver

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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.