class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        #
        # NOTE: **READ THE PROBLEM DESCRIPTION ALWAYS!!**
        #
        # Even though it mentions Sudoku, it doesn't mean the
        # rules are the same as the real world. Which is the case
        # here — we need to only look at the filled cell and the rules
        # provided.
        
        size = len(board)
        
        # 1. Check if rows and cols are valid
        #
        # We can iterate both rows and cols at the same time since
        # the sudoku board has 9 rows and 9 cols
        for i in range(size):
            row = set()
            col = set()
            
            for j in range(size):
                # ith row check
                if board[i][j] != '.':
                    if board[i][j] in row:
                        return False
                    
                    row.add(board[i][j])
                
                # ith col check
                if board[j][i] != '.':
                    if board[j][i] in col:
                        return False
                    
                    col.add(board[j][i])
        
        # 2. Check if squares are valid
        # 
        # We can do normal row * col iteration and use `// 3` to figure
        # out which square a cell belongs to
        
        squares = [[set(), set(), set()] for _ in range(3)]
        for i in range(size):
            for j in range(size):
                cell = board[i][j]
                if cell == '.':
                    continue
                
                square = squares[i // 3][j // 3]
                if cell in square:
                    return False
                
                square.add(cell)
        
        return True