SHELL希尔排序

 /******************************************************************************
  *  Compilation:  javac Shell.java
  *  Execution:    java Shell < input.txt
  *  Dependencies: StdOut.java StdIn.java
  *  Data files:   http://algs4.cs.princeton.edu/21elementary/tiny.txt
  *                http://algs4.cs.princeton.edu/21elementary/words3.txt
  *
  *  Sorts a sequence of strings from standard input using shellsort.
  *
  *  Uses increment sequence proposed by Sedgewick and Incerpi.
  *  The nth element of the sequence is the smallest integer >= 2.5^n
  *  that is relatively prime to all previous terms in the sequence.
  *  For example, incs[4] is 41 because 2.5^4 = 39.0625 and 41 is
  *  the next integer that is relatively prime to 3, 7, and 16.
  *
  *  % more tiny.txt
  *  S O R T E X A M P L E
  *
  *  % java Shell < tiny.txt
  *  A E E L M O P R S T X                 [ one string per line ]
  *
  *  % more words3.txt
  *  bed bug dad yes zoo ... all bad yet
  *
  *  % java Shell < words3.txt
  *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
  *
  *
  ******************************************************************************/

 package edu.princeton.cs.algs4;

 /**
  *  The {@code Shell} class provides static methods for sorting an
  *  array using Shellsort with Knuth's increment sequence (1, 4, 13, 40, ...).
  *  <p>
  *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
  *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
  *
  *  @author Robert Sedgewick
  *  @author Kevin Wayne
  */
 public class Shell {

     private 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);
                 }
             }
             assert isHsorted(a, h);
             h /= 3;
         }
         assert isSorted(a);
     }

     private static boolean less(Comparable v, Comparable w) {
         return v.compareTo(w) < 0;
     }

     private static void exch(Object[] a, int i, int j) {
         Object swap = a[i];
         a[i] = a[j];
         a[j] = swap;
     }

     private static boolean isSorted(Comparable[] a) {
         for (int i = 1; i < a.length; i++)
             if (less(a[i], a[i-1])) return false;
         return true;
     }

     private static boolean isHsorted(Comparable[] a, int h) {
         for (int i = h; i < a.length; i++)
             if (less(a[i], a[i-h])) return false;
         return true;
     }

     private static void show(Comparable[] a) {
         for (int i = 0; i < a.length; i++) {
             StdOut.println(a[i]);
         }
     }

     public static void main(String[] args) {
         String[] a = StdIn.readAllStrings();
         Shell.sort(a);
         show(a);
     }

 }