LeetCode 36 - Valid Sudoku - 2023.01.03 solution

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        """ How to solve it: Iterate through the cells once, maintaining three different dicts of sets,
        one dict for the rows, one dict for the columns, and one for the squares.  If you hit a duplicate,
        return False, otherwise if you get to the end return True.
        """
        row_sets = defaultdict(set)
        col_sets = defaultdict(set)
        square_sets = defaultdict(set)

        for r in range(9):
            for c in range(9):
                if board[r][c] == '.':
                    continue
                if board[r][c] in col_sets[c] or board[r][c] in row_sets[r] or \
                   board[r][c] in square_sets[(r//3, c//3)]:
                    return False
                else:
                    row_sets[r].add(board[r][c])
                    col_sets[c].add(board[r][c])
                    square_sets[(r//3, c//3)].add(board[r][c])

        return True