SHELL希尔排序

  1 /******************************************************************************
  2  *  Compilation:  javac Shell.java
  3  *  Execution:    java Shell < input.txt
  4  *  Dependencies: StdOut.java StdIn.java
  5  *  Data files:   http://algs4.cs.princeton.edu/21elementary/tiny.txt
  6  *                http://algs4.cs.princeton.edu/21elementary/words3.txt
  7  *   
  8  *  Sorts a sequence of strings from standard input using shellsort.
  9  *
 10  *  Uses increment sequence proposed by Sedgewick and Incerpi.
 11  *  The nth element of the sequence is the smallest integer >= 2.5^n
 12  *  that is relatively prime to all previous terms in the sequence.
 13  *  For example, incs[4] is 41 because 2.5^4 = 39.0625 and 41 is
 14  *  the next integer that is relatively prime to 3, 7, and 16.
 15  *   
 16  *  % more tiny.txt
 17  *  S O R T E X A M P L E
 18  *
 19  *  % java Shell < tiny.txt
 20  *  A E E L M O P R S T X                 [ one string per line ]
 21  *    
 22  *  % more words3.txt
 23  *  bed bug dad yes zoo ... all bad yet
 24  *  
 25  *  % java Shell < words3.txt
 26  *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 27  *
 28  *
 29  ******************************************************************************/
 30 
 31 package edu.princeton.cs.algs4;
 32 
 33 /**
 34  *  The {@code Shell} class provides static methods for sorting an
 35  *  array using Shellsort with Knuth's increment sequence (1, 4, 13, 40, ...).
 36  *  <p>
 37  *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
 38  *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 39  *  
 40  *  @author Robert Sedgewick
 41  *  @author Kevin Wayne
 42  */
 43 public class Shell {
 44 
 45   
 46     private Shell() { }
 47 
 48  
 52     public static void sort(Comparable[] a) {
 53         int n = a.length;
 54 
 55       
 56         int h = 1;
 57         while (h < n/3) h = 3*h + 1; 
 58 
 59         while (h >= 1) {
 60         
 61             for (int i = h; i < n; i++) {
 62                 for (int j = i; j >= h && less(a[j], a[j-h]); j -= h) {
 63                     exch(a, j, j-h);
 64                 }
 65             }
 66             assert isHsorted(a, h); 
 67             h /= 3;
 68         }
 69         assert isSorted(a);
 70     }
 71 
 72 
 73 
 77     
 78    
 79     private static boolean less(Comparable v, Comparable w) {
 80         return v.compareTo(w) < 0;
 81     }
 82         
 83   
 84     private static void exch(Object[] a, int i, int j) {
 85         Object swap = a[i];
 86         a[i] = a[j];
 87         a[j] = swap;
 88     }
 89 
 90 
 91   
 94     private static boolean isSorted(Comparable[] a) {
 95         for (int i = 1; i < a.length; i++)
 96             if (less(a[i], a[i-1])) return false;
 97         return true;
 98     }
 99 
100 
101     private static boolean isHsorted(Comparable[] a, int h) {
102         for (int i = h; i < a.length; i++)
103             if (less(a[i], a[i-h])) return false;
104         return true;
105     }
106 
107   
108     private static void show(Comparable[] a) {
109         for (int i = 0; i < a.length; i++) {
110             StdOut.println(a[i]);
111         }
112     }
113 
114     
120     public static void main(String[] args) {
121         String[] a = StdIn.readAllStrings();
122         Shell.sort(a);
123         show(a);
124     }
125 
126 }
127