leetcode 36 - valid sudoku.py

# valid sudoku | leetcode 36 | https://leetcode.com/problems/valid-sudoku/
# Determine if a n^2 x n^2 Psuedo-Sudoku board is valid. 
# Only the filled cells need to be validated.

import collections

class Solution:
    def isValidSudoku(self, board: list[list[str]]) -> bool:
        rows = collections.defaultdict(set)
        columns = collections.defaultdict(set)
        squares = collections.defaultdict(set)
        
        for i in range(len(board)):
            for j in range(len(board[i])):
                
                if board[i][j] == '.':
                    continue
                
                isInRow = board[i][j] in rows[i]
                isInColumn = board[i][j] in columns[j]
                isInSquare = board[i][j] in squares[(i//3, j//3)]
                if (isInRow or isInColumn or isInSquare):
                    return False
                
                rows[i].add(board[i][j])
                columns[j].add(board[i][j])
                squares[(i//3, j//3)].add(board[i][j])
        
        return True