Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’.

A partially filled sudoku which is valid.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

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public class Solution {
    public boolean isValidSudoku(char[][] board) {
        if (board == null || board.length != 9 || board[0].length != 9) {
            return false;
        }
        for (int i = 0; i < 9; i++) {
            HashSet<Character> row = new HashSet<>();
            HashSet<Character> col = new HashSet<>();
            HashSet<Character> cube = new HashSet<>();
            for (int j = 0; j < 9; j++) {
                if (board[i][j] != '.' && !row.add(board[i][j]))
                    return false;
                if (board[j][i] != '.' && !col.add(board[j][i]))
                    return false;
                int rowIndex = 3 * (i / 3);
                int colIndex = 3 * (i % 3);
                if (board[rowIndex + j / 3][colIndex + j % 3] != '.' && !cube.add(board[rowIndex + j / 3][colIndex + j % 3]))
                    return false;
            }
        }
        return true;
    }
}