public static String gridChallenge(List<String> grid) {
        int[][] values = new int[grid.size()][grid.get(0).length()];
        for (int i=0 ; i<grid.size() ; i++) {
            char[] chars = grid.get(i).toCharArray();
            Arrays.sort(chars);
            
            for (int j=0 ; j<chars.length ; j++) {
                values[i][j] = (int) chars[j];
                if (i > 0 && values[i-1][j] > values[i][j]) {
                     return "NO";
                }
            }     
        }
        return "YES";
    }