Valid Sudoku (leetcode)

public boolean isValidSudoku(char[][] board) {
        Set<String> set = new HashSet<>();

        for(int i=0; i<board.length; i++){
            for(int j=0; j<board[0].length; j++){
                char ch = board[i][j];
                if(ch!='.'){
                    if(!set.add("row" + i + ch) || !set.add("col" + j + ch) || !set.add("grid" + ch + i/3 + j/3)){
                        return  false;
                    }
                }
            }
        }
        return true;
    }