valid sudoku leetcode solution in java code example

Example: valid sudoku leetcode solution in java

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;
    }
}