[LC] 36. Valid Sudoku

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

1. Each row must contain the digits 1-9 without repetition.
2. Each column must contain the digits 1-9 without repetition.
3. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

class Solution {
    public boolean isValidSudoku(char[][] board) {
        int n = board.length;
        for (int i = 0; i < n; i++) {
            Set<Character> rowSet = new HashSet<>();
            Set<Character> colSet = new HashSet<>();
            Set<Character> cubeSet = new HashSet<>();
            for (int j = 0; j < n; j++) {
                if (board[i][j] != '.' && !rowSet.add(board[i][j])) {
                    return false;
                }
                if (board[j][i] != '.' && !colSet.add(board[j][i])) {
                    return false;
                }
                int rowIndex = 3 * (i / 3);
                int colIndex = 3 * (i % 3);
                
                if (board[rowIndex + j / 3][colIndex + j % 3] != '.' && !cubeSet.add(board[rowIndex + j / 3][colIndex + j % 3])) {
                    return false;
                }
            }
        }
        return true;
    }
}