leetcode “36. valid sudoku”

class Solution {
    public boolean isValidSudoku(char[][] board) {
        Map<Integer, Set<Character>> rowMap = new HashMap<>();
        Map<Integer, Set<Character>> colMap = new HashMap<>();
        Map<Integer, Set<Character>> blockMap = new HashMap<>();
        for (int i = 0; i < 9; i++) {
            rowMap.put(i, new HashSet<Character>());
            colMap.put(i, new HashSet<Character>());
            blockMap.put(i, new HashSet<Character>());
        }
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                char c = board[i][j];
                if (c == '.') {
                    continue;
                }
                int block = (int)(i / 3) * 3 + j / 3;
                Set<Character> rowSet = rowMap.get(i);
                Set<Character> colSet = colMap.get(j);
                Set<Character> blockSet = blockMap.get(block);
                if (rowSet.contains(c) || colSet.contains(c) || blockSet.contains(c)) {
                    return false;
                }
                rowSet.add(c);
                colSet.add(c);
                blockSet.add(c);
            }
        }
        return true;
    }
}