solution.py

from typing import List
import collections

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        cols = collections.defaultdict(set)
        rows = collections.defaultdict(set)
        squares = collections.defaultdict(set)

        for r in range(9):
            for c in range(9):
                if board[r][c] == ".":
                    continue
                if (board[r][c] in rows[r] or
                    board[r][c] in cols[c] or
                        board[r][c] in squares[(r//3, c//3)]):
                    return False
                cols[c].add(board[r][c])
                rows[r].add(board[r][c])
                squares[(r//3, c//3)].add(board[r][c])
        return True


if __name__ == '__main__':
    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"]]
print(Solution().isValidSudoku(board))