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

 package algorithms.mergesort22;

 import algorithms.util.StdIn;
 import algorithms.util.StdOut;

 /******************************************************************************
  *  Compilation:  javac MergeBU.java
  *  Execution:    java MergeBU < input.txt
  *  Dependencies: StdOut.java StdIn.java
  *  Data files:   http://algs4.cs.princeton.edu/22mergesort/tiny.txt
  *                http://algs4.cs.princeton.edu/22mergesort/words3.txt
  *
  *  Sorts a sequence of strings from standard input using
  *  bottom-up mergesort.
  *
  *  % more tiny.txt
  *  S O R T E X A M P L E
  *
  *  % java MergeBU < 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 MergeBU < words3.txt
  *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
  *
  ******************************************************************************/

 /**
  *  The <tt>MergeBU</tt> class provides static methods for sorting an
  *  array using bottom-up mergesort.
  *  <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 MergeBU {

     // This class should not be instantiated.
     private MergeBU() { }

     // stably merge a[lo..mid] with a[mid+1..hi] using aux[lo..hi]
     private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) {

         // copy to aux[]
         for (int k = lo; k <= hi; k++) {
             aux[k] = a[k];
         }

         // merge back to a[]
         int i = lo, j = mid+1;
         for (int k = lo; k <= hi; k++) {
             if      (i > mid)              a[k] = aux[j++];  // this copying is unneccessary
             else if (j > hi)               a[k] = aux[i++];
             else if (less(aux[j], aux[i])) a[k] = aux[j++];
             else                           a[k] = aux[i++];
         }

     }

     /**
      * Rearranges the array in ascending order, using the natural order.
      * @param a the array to be sorted
      */
     public static void sort(Comparable[] a) {
         int N = a.length;
         Comparable[] aux = new Comparable[N];
         for (int n = 1; n < N; n = n+n) {
             for (int i = 0; i < N-n; i += n+n) {
                 int lo = i;
                 int m  = i+n-1;
                 int hi = Math.min(i+n+n-1, N-1);
                 merge(a, aux, lo, m, hi);
             }
         }
         assert isSorted(a);
     }

   /***********************************************************************
     *  Helper sorting functions.
     ***************************************************************************/

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

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

    /***************************************************************************
     *  Check if array is sorted - useful for debugging.
     ***************************************************************************/
     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;
     }

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

     /**
      * Reads in a sequence of strings from standard input; bottom-up
      * mergesorts them; and prints them to standard output in ascending order.
      */
     public static void main(String[] args) {
         String[] a = StdIn.readAllStrings();
         MergeBU.sort(a);
         show(a);
     }
 }