36.py

#Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

#Each row must contain the digits 1-9 without repetition.
#Each column must contain the digits 1-9 without repetition.
#Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.
#Note:

#A Sudoku board (partially filled) could be valid but is not necessarily solvable.
#Only the filled cells need to be validated according to the mentioned rules.

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        row = collections.defaultdict(set)
        col = collections.defaultdict(set)
        sqr = collections.defaultdict(set)

        for r in range(9):
            for c in range(9):
                if board[r][c] == ".":
                    continue
                
                if board[r][c] in row[r] or board[r][c] in col[c] or board[r][c] in sqr[(r//3, c//3)]:
                    return False
                
                row[r].add(board[r][c])
                col[c].add(board[r][c])
                sqr[(r//3, c//3)].add(board[r][c])

        return True