Valid Sudoku - LeetCode

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

1. Each row must contain the digits 1-9 without repetition.
2. Each column must contain the digits 1-9 without repetition.
3. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

A partially filled sudoku which is valid.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

Example 1:

Input:
[
  ["5","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
Output: true

Example 2:

Input:
[
  ["8","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
Output: false
Explanation: Same as Example 1, except with the 5 in the top left corner being 
    modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.

Note:

• A Sudoku board (partially filled) could be valid but is not necessarily solvable.
• Only the filled cells need to be validated according to the mentioned rules.
• The given board contain only digits 1-9 and the character '.'.
• The given board size is always 9x9.

Java Solution :

class Solution {
    public boolean isValidSudoku(char[][] board) {
        if(board == null || board.length == 0)
            return false;
        HashMap<Integer,Integer> [] rows = new HashMap[9];
        HashMap<Integer,Integer> [] cols = new HashMap[9];
        HashMap<Integer,Integer> [] boxes = new HashMap[9];
        
        for(int i = 0; i < 9 ; i++){
            rows[i] = new HashMap<Integer,Integer>();
            cols[i] = new HashMap<Integer,Integer>();
            boxes[i] = new HashMap<Integer,Integer>();
        }
        int num =0;
        for(int i =0 ; i < 9 ; i++){
            for(int j=0; j < 9; j++){
                char n = board[i][j];
                if(board[i][j] != '.'){
                 num = (int)n;
                int box_index = (i/3)*3 + j/3;
                rows[i].put(num,rows[i].getOrDefault(num,0)+1);
                cols[j].put(num,cols[j].getOrDefault(num,0)+1);
                boxes[box_index].put(num,boxes[box_index].getOrDefault(num,0)+1);
                
                if(rows[i].get(num) > 1 || cols[j].get(num) > 1 || boxes[box_index].get(num) > 1)
                    return false;
            }
        }
        }
        
        return true;
    }
}