Determine if a Sudoku is Valid

Determine if a Sudoku is valid
The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’.

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

Because the number range of sudoku is 1-9, so each number in each row, col and block should be unique, then we can go through every position of given board, check if the number has been
Found in current row,current column and current block. If so, return false.

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        for(int i=0; i<9; i++){
            for(int j=0; j<9; j++){
                if(!isValid(board,i,j)){
                    return false;
                }
            }
        }
        return true;
        
    }
    
     private boolean isValid(char[][] board, int a, int b) {
		HashSet<Character> contained = new HashSet<Character>();
		// cache the occurrence and return false in case of a duplicate
		for (int i = 0; i < 9; i++) {
			if (contained.contains(board[a][i])) {
				return false;
			}
			if (board[a][i] > '0' && board[a][i] <= '9') {
				contained.add(board[a][i]);
			}
		}
		
		
		
		contained.clear();
		for (int i = 0; i < 9; i++) {
			if (contained.contains(board[i][b])) {
				return false;
			}
			if (board[i][b] > '0' && board[i][b] <= '9') {
				contained.add(board[i][b]);
			}
		}
		
		// Check sub-box board[a][b] located.
		contained.clear();
		for (int i = 0; i < 3; i++) {
			for (int j = 0; j < 3; j++) {
				int x = a / 3 * 3 + i, y = b / 3 * 3 + j;
				if (contained.contains(board[x][y])) {
					return false;
				}
				
				if (board[x][y] > '0' && board[x][y] <= '9') {
					contained.add(board[x][y]);
				}
			}
		}		
		return true;
	}
}