What is the error in this code


#1

public class Solution {
// DO NOT MODIFY THE LIST. IT IS READ ONLY
public int isValidSudoku(final List A) {
Map<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>();
}

// validate a board
for (int i = 0; i < A.size() ; i++) {
  
    String num = A.get(i);
    if (!num.equals(".")) {
      int n = Integer.parseInt(num);
      int row= i/9;
      int col =i%9;
      int box_index = (row / 3 ) * 3 + col / 3;
        
      // keep the current cell value
      rows[row].put(n, rows[row].getOrDefault(n, 0) + 1);
      columns[col].put(n, columns[col].getOrDefault(n, 0) + 1);
      boxes[box_index].put(n, boxes[box_index].getOrDefault(n, 0) + 1);

      // check if this value has been already seen before
      if (rows[row].get(n) > 1 || columns[col].get(n) > 1 || boxes[box_index].get(n) > 1)
        return 0;
    }
  
}

return 1;
}

}