2.1.9

question:

Show, in the style of the example trace with ALGORITHM 2.3, how shellsort sorts the array E A S Y S H E L L S O R T Q U E S T I O N.

answer:

import edu.princeton.cs.algs4.*;

public class Shell
{
    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);
            }
            StdOut.print(h + "-sort\t");
            for(int t = 0; t < a.length; t++)
                StdOut.print(a[t] + " ");
            StdOut.println();
            h = h/3;
        }
    }
    
    private static boolean less(Comparable v, Comparable w)
    {
        return v.compareTo(w) < 0;
    }
    
    private static void exch(Comparable[] a, int i, int j)
    {
        Comparable t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
    
    private static void show(Comparable[] a)
    {
        for(int i = 0; i < a.length; i++)
            StdOut.print(a[i] + " ");
        StdOut.println();
    }
    
    public static boolean isSorted(Comparable[] a)
    {
        for(int i = 1; i < a.length; i++)
            if(less(a[i], a[i-1])) return false;
        return true;
    }
    
    public static void main(String[] args)
    {
        //输入E A S Y S H E L L S O R T Q U E S T I O N
        String[] a = In.readStrings();//CTRL + d
        sort(a);
        assert isSorted(a);
        show(a);
    }
}