Leetcode-Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

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

A partially filled sudoku which is valid.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

Solution:

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        int rowNum = board.length;
        if (rowNum!=9) return false;
        int colNum = board[0].length;
        if (colNum!=9) return false;

        for (int i=0;i<rowNum;i++)
            if (!checkArea(board,i,0,1,9)) return false;
  
        for (int i=0;i<colNum;i++)
            if (!checkArea(board,0,i,9,1)) return false;

        int[] index = new int[]{0,3,6};
        for (int i=0;i<3;i++)
            for (int j=0;j<3;j++)
                if (!checkArea(board,index[i],index[j],3,3)) return false;

        return true;
       
        
    }

    public boolean checkArea(char[][] board, int x, int y, int rowNum, int colNum){
        Set<Integer> set = new HashSet<Integer>();
        for (int i=0;i<rowNum;i++)
            for (int j=0;j<colNum;j++)
                if (board[x+i][y+j]!='.'){
                    int val = board[x+i][y+j]-'0';
                    if (val<0 || val>9) return false;
                    if (set.contains(val))
                        return false;
                    else set.add(val);
                }

        return true;

    }
                
}