class Solution {
    public boolean isValidSudoku(char[][] board) {
        HashSet<Character> myHashSet = new HashSet<>();
        //rows
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] != '.') {
                    if (!myHashSet.contains(board[i][j])) {
                        myHashSet.add(board[i][j]);
                    } else {
                        return false;
                    }
                }
            }
            myHashSet.clear();
        }
        //columns
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[j][i] != '.') {
                    if (!myHashSet.contains(board[j][i])) {
                        myHashSet.add(board[j][i]);
                    } else {
                        return false;
                    }
                }
            }
            myHashSet.clear();
        }
        //3x3
        for (int i = 0; i < 9; i += 3) {
            for (int j = 0; j < 9; j += 3) {
                for (int m = i; m < i+2; m++) {
                    for (int n = j; n < j+2; n++) {
                        if (board[m][n] != '.') {
                            if (!myHashSet.contains(board[m][n])) {
                                myHashSet.add(board[m][n]);
                            } else {
                                return false;
                            }
                        }
                    }
                    myHashSet.clear();
                }
            }
        }
        return true;
    }
}