public boolean isValidSudoku(char[][] board) {

// init data

HashMap<Integer, Integer>[] rows = new HashMap[9];

HashMap<Integer, Integer> [] columns = new HashMap[9];

HashMap<Integer, Integer> [] boxes = new HashMap[9];

for (int i = 0; i < 9; i++) {

rows[i] = new HashMap<Integer, Integer>();

columns[i] = new HashMap<Integer, Integer>();

boxes[i] = new HashMap<Integer, Integer>();

}

for (int i = 0; i < 9; i++) {

for (int j = 0; j < 9; j++) {

char num = board[i][j];

if (num != '.') {

int n = (int)num;

int box_index = (i / 3 ) * 3 + j / 3;

rows[i].put(n, rows[i].getOrDefault(n, 0) + 1);

columns[j].put(n, columns[j].getOrDefault(n, 0) + 1);

boxes[box_index].put(n, boxes[box_index].getOrDefault(n, 0) + 1);

if (rows[i].get(n) > 1 || columns[j].get(n) > 1 || boxes[box_index].get(n) > 1)

return false;

}

}

}

return true;

}