sudoku-solver

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

Each of the digits 1-9 must occur exactly once in each row.
Each of the digits 1-9 must occur exactly once in each column.
Each of the the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
Empty cells are indicated by the character '.'.


A sudoku puzzle...


...and its solution numbers marked in red.

Note:

The given board contain only digits 1-9 and the character '.'.
You may assume that the given Sudoku puzzle will have a single unique solution.
The given board size is always 9x9.

My code:
class Solution:
    def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        self.board=board
        self.nums = {chr(i + ord('1')) for i in range(9)}
        self.rows = {i : set() for i in range(9)}
        self.cols = {i : set() for i in range(9)}
        self.sqrs = {(i,j) : set() for i in range(3) for j in range(3)} 
        for i, row in enumerate(board):
            for j, col in enumerate(row):
                if col == '.': continue
                self.rows[i].add(col)
                self.cols[j].add(col)
                self.sqrs[(i//3,j//3)].add(col)
        self.flag=False
        self.find(0,0)
    def add(self,i,j,n):
        col=n
        self.rows[i].add(col)
        self.cols[j].add(col)
        self.sqrs[(i//3,j//3)].add(col)
        self.board[i][j] = n
    def rm(self,i,j,n):
        col=n
        self.rows[i].discard(col)
        self.cols[j].discard(col)
        self.sqrs[(i//3,j//3)].discard(col)
        self.board[i][j] = '.'
    def options(self, i, j):
        return self.nums - self.rows[i] - self.cols[j] - self.sqrs[(i//3,j//3)]
    def find(self,i,j):
        if j>8:
            return self.find(i+1,0)
        if i>8:
            self.flag=True
        else:
            if self.board[i][j] != '.': return self.find(i, j+1)
            opts = self.options(i, j)
            if not opts: return
            for n in opts:
                self.add(i, j, n)
                self.find(i, j+1)
                if self.flag: return
                self.rm(i, j, n)
        
        
        
        
        
Solution:
https://leetcode.com/problems/sudoku-solver