human
I found an interesting problem on https://leetcode.com/problems/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.

I tried it in Python, but could not do it. Can you solve it?
assistant
#Here is the Python code you requested.

class Solution:
     def reduceSingleOption(self, board, emptyGrid, valid):
         m = 1
         while m == 1:
             m = 10
             for k in range(len(emptyGrid) - 1, -1, -1):
                 i, j = emptyGrid[k]
                 option = set.intersection(valid[i],
                                           valid[j + 9],
                                           valid[(i // 3) * 3 + j // 3 + 18])
                 if not option:
                     return
                 if len(option) < m:
                     m = len(option)
                 if len(option) == 1:
                     option = option.pop()
                     board[i][j] = str(option)
                     emptyGrid.pop(k)
                     valid[i].remove(option)
                     valid[j + 9].remove(option)
                     valid[(i // 3) * 3 + j // 3 + 18].remove(option)
 
     def dfs(self, board, emptyGrid, valid, ans):
         if len(ans) == len(emptyGrid):
             return
         i, j = emptyGrid[len(ans)]
         options = set.intersection(valid[i],
                                    valid[j + 9],
                                    valid[(i // 3) * 3 + j // 3 + 18])
         if len(options) == 0:
             return
         else:
             for temp in options:
                 print(i, j, temp)
                 ans.append(temp)
                 valid[i].remove(temp)
                 valid[j + 9].remove(temp)
                 valid[(i // 3) * 3 + j // 3 + 18].remove(temp)
                 self.dfs(board, emptyGrid, valid, ans)
                 if len(ans) == len(emptyGrid):
                     break
                 temp = ans.pop()
                 valid[i].add(temp)
                 valid[j + 9].add(temp)
                 valid[(i // 3) * 3 + j // 3 + 18].add(temp)
 
 
     def solveSudoku(self, board):
         """
         :type board: List[List[str]]
         :rtype: void Do not return anything, modify board in-place instead.
         """
         valid = [set(range(1, 10)) for _ in...