Java //O(n*n)

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