public static void sort(Comparable[] a) {
        int N = a.length;
        int h = 1;
        
        while (h < N/3) {
            h = 3*h + 1;
        }
        
        while (h >= 1) {
            for (int i = h; i < N; i++) {
                for (int j = i; j >= h && less(a[j], a[j-h]); j-= h) {
                    exch(a, j, j-h);
                }
            }
            h = h/3;
        }
    }