26. Valid Sudoku

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.

Example 1:

Input: 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"]]
Output: true

Example 2:

Input: board =
[["8","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"]]
Output: false
Explanation: Same as Example 1, except with the 5 in the top left corner being modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.

def isValidSudoku(self, board: List[List[str]]) -> bool:

        for i in range(9):
            row_set = set()
            col_set = set()
            for j in range(9):
                # Check rows
                if board[i][j] != '.':
                    if board[i][j] in row_set:
                        return False
                    row_set.add(board[i][j])  
                # Check columns
                if board[j][i] != '.':
                    if board[j][i] in col_set:
                        return False
                    col_set.add(board[j][i])

        for i in range(0, 9, 3):
            for j in range(0, 9, 3):
                box_set = set()
                for k in range(3):
                    for l in range(3):
                        if board[i + k][j + l] != '.':
                            if board[i + k][j + l] in box_set:
                                return False
                            box_set.add(board[i + k][j + l])

        return True