算法Sedgewick第四版-第1章基础-2.1Elementary Sortss-007归并排序(自下而上)

一、

1.

2.

3.

二、代码

  1 package algorithms.mergesort22;
  2 
  3 import algorithms.util.StdIn;
  4 import algorithms.util.StdOut;
  5 
  6 /******************************************************************************
  7  *  Compilation:  javac MergeBU.java
  8  *  Execution:    java MergeBU < input.txt
  9  *  Dependencies: StdOut.java StdIn.java
 10  *  Data files:   http://algs4.cs.princeton.edu/22mergesort/tiny.txt
 11  *                http://algs4.cs.princeton.edu/22mergesort/words3.txt
 12  *   
 13  *  Sorts a sequence of strings from standard input using
 14  *  bottom-up mergesort.
 15  *   
 16  *  % more tiny.txt
 17  *  S O R T E X A M P L E
 18  *
 19  *  % java MergeBU < 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 MergeBU < words3.txt
 26  *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 27  *
 28  ******************************************************************************/
 29 
 30 /**
 31  *  The <tt>MergeBU</tt> class provides static methods for sorting an
 32  *  array using bottom-up mergesort.
 33  *  <p>
 34  *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
 35  *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 36  *
 37  *  @author Robert Sedgewick
 38  *  @author Kevin Wayne
 39  */
 40 public class MergeBU {
 41 
 42     // This class should not be instantiated.
 43     private MergeBU() { }
 44 
 45     // stably merge a[lo..mid] with a[mid+1..hi] using aux[lo..hi]
 46     private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) {
 47 
 48         // copy to aux[]
 49         for (int k = lo; k <= hi; k++) {
 50             aux[k] = a[k]; 
 51         }
 52 
 53         // merge back to a[]
 54         int i = lo, j = mid+1;
 55         for (int k = lo; k <= hi; k++) {
 56             if      (i > mid)              a[k] = aux[j++];  // this copying is unneccessary
 57             else if (j > hi)               a[k] = aux[i++];
 58             else if (less(aux[j], aux[i])) a[k] = aux[j++];
 59             else                           a[k] = aux[i++];
 60         }
 61 
 62     }
 63 
 64     /**
 65      * Rearranges the array in ascending order, using the natural order.
 66      * @param a the array to be sorted
 67      */
 68     public static void sort(Comparable[] a) {
 69         int N = a.length;
 70         Comparable[] aux = new Comparable[N];
 71         for (int n = 1; n < N; n = n+n) {
 72             for (int i = 0; i < N-n; i += n+n) {
 73                 int lo = i;
 74                 int m  = i+n-1;
 75                 int hi = Math.min(i+n+n-1, N-1);
 76                 merge(a, aux, lo, m, hi);
 77             }
 78         }
 79         assert isSorted(a);
 80     }
 81 
 82   /***********************************************************************
 83     *  Helper sorting functions.
 84     ***************************************************************************/
 85     
 86     // is v < w ?
 87     private static boolean less(Comparable v, Comparable w) {
 88         return v.compareTo(w) < 0;
 89     }
 90 
 91    // exchange a[i] and a[j]
 92     private static void exch(Object[] a, int i, int j) {
 93         Object swap = a[i];
 94         a[i] = a[j];
 95         a[j] = swap;
 96     }
 97 
 98 
 99    /***************************************************************************
100     *  Check if array is sorted - useful for debugging.
101     ***************************************************************************/
102     private static boolean isSorted(Comparable[] a) {
103         for (int i = 1; i < a.length; i++)
104             if (less(a[i], a[i-1])) return false;
105         return true;
106     }
107 
108     // print array to standard output
109     private static void show(Comparable[] a) {
110         for (int i = 0; i < a.length; i++) {
111             StdOut.println(a[i]);
112         }
113     }
114 
115     /**
116      * Reads in a sequence of strings from standard input; bottom-up
117      * mergesorts them; and prints them to standard output in ascending order. 
118      */
119     public static void main(String[] args) {
120         String[] a = StdIn.readAllStrings();
121         MergeBU.sort(a);
122         show(a);
123     }
124 }

 

posted @ 2016-04-26 11:24  shamgod  阅读(455)  评论(0)  编辑  收藏  举报
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