Sudoku Solver

class Solution:
    def solveSudoku(self, board):
        def solve():
            for row in range(9):
                for col in range(9):
                    if board[row][col] == '.':
                        for num in '123456789':
                            if is_valid(row, col, num):
                                board[row][col] = num
                                if solve():
                                    return True
                                board[row][col] = '.'
                        return False
            return True

        def is_valid(row, col, num):
            for i in range(9):
                if board[row][i] == num or board[i][col] == num:
                    return False
            start_row, start_col = 3 * (row // 3), 3 * (col // 3)
            for i in range(start_row, start_row + 3):
                for j in range(start_col, start_col + 3):
                    if board[i][j] == num:
                        return False
            return True

        solve()