Now, I am working on matrix problems.

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Solution

This is my initial greedy solution.

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        for row in board:
            rowVals = set()
            for column in row:
                if column != '.' and column in rowVals:
                    return False
                rowVals.add(column)

        for col in range(len(board[0])):
            colVals = set()
            for row in board:
                if row[col] != '.' and row[col] in colVals:
                    return False
                colVals.add(row[col])

        for subX in range(3):
            for subY in range(3):
                gridVals = set()
                for x in range(3):
                    for y in range(3):
                        cell = board[subY * 3 + y][subX * 3 + x]
                        if cell != '.' and cell in gridVals:
                            return False
                        gridVals.add(board[subY * 3 + y][subX * 3 + x])
        
        return True

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Improvements

Now, I reviewed other solutions and liked this one which uses a list of tuples and only one nested for loop.

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        vals = []
        for i, row in enumerate(board):
            for j, x in enumerate(row):
                if x != '.':
                    vals += [(x, j), (i, x), (x, i // 3, j // 3)]
        return len(vals) == len(set(vals))

I like how tuples are made unique by swapping their position, and the cell value is not mistaken for the row/column because it is a string rather than int. O(n) = 1 since board size is fixed.