LeetCode Valid Sudoku

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.

Sudoku规则：

在一个9*9的区域内，

每行1-9出现且只出现一次，

每列1-9出现且只出现一次，

在9个子3*3的区域内1-9出现且只出现一次。

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        ArrayList<HashSet<Integer>> row = new ArrayList<HashSet<Integer>>();
        ArrayList<HashSet<Integer>> col = new ArrayList<HashSet<Integer>>();
        ArrayList<HashSet<Integer>> box = new ArrayList<HashSet<Integer>>();

        for (int i = 0; i < 9; i++) {
            row.add(new HashSet<Integer>());
            col.add(new HashSet<Integer>());
            box.add(new HashSet<Integer>());
        }

        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') {
                    continue;
                }
                if (row.get(i).contains(board[i][j]-'0')) {
                    return false;
                } else row.get(i).add(board[i][j]-'0');

                if (col.get(j).contains(board[i][j] - '0')) {
                    return  false;
                }else col.get(j).add(board[i][j] - '0');

                int index = ((i / 3) * 3) + j / 3;
                if (box.get(index).contains(board[i][j] - '0')) {
                    return false;
                }else box.get(index).add(board[i][j] - '0');
            }
        }

        return true;
    }
}