listmergesort.java

Java数据结构开发包

/*
  Copyright (c) 1999, 2000 Brown University, Providence, RI
  
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  copyright notice and this permission notice appear in supporting
  documentation, and that the name of Brown University not be used in
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  without specific, written prior permission.
  
  BROWN UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
  SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
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  UNIVERSITY BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL
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  TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
  PERFORMANCE OF THIS SOFTWARE.
*/

package jdsl.core.algo.sorts;

import jdsl.core.api.*;


/**
 * Performs a merge-sort in O(n log n) time, provided that isEmpty(),
 * first(), insertLast(), and removeFirst() all operate in O(1) time.
 *
 * @author Benoit Hudson
 * @author Roberto Tamassia
 * @author Luca Vismara
 * @author Keith Schmidt
 * @version JDSL 2.1.1 
 */
public class ListMergeSort implements SortObject {

  
  /** 
   * Recursively divides a Sequence into (roughly) equal subsequences
   * and merges them back together once sorted.
   *
   * @param S the sequence to sort
   * @param c the comparator to use in the sort 
   */
  public void sort(Sequence S, Comparator c) {

    int n = S.size();
    if (n > 1) {

      // divide
      Sequence S1 = (Sequence)S.newContainer();
      for (int i=1; i <= (n+1)/2; i++) {
	S1.insertLast(S.removeFirst());
      }
      Sequence S2 = (Sequence)S.newContainer();
      for (int i=1; i <= n/2; i++) {
	S2.insertLast(S.removeFirst());
      }

      // recur
      sort(S1,c);
      sort(S2,c);

      //conquer
      merge(S1,S2,c,S);
    }
  }

  
  /** 
   * Merges two sequences.
   * This is done by taking the lower of the two sequence's first
   * elements off the appropriate sequence and into the sequence to
   * return.  This is done until one of the sequences is empty, at which
   * point the remainder of the other sequence is appended, and the
   * sequence returned.  Note that in this implementation, S1 and S2 are
   * destroyed.  
   *
   * @param S1 the first sequence to merge
   * @param S2 the second sequence to merge
   * @param c the comparator to use
   * @param S the sequence that will store the merge of S1 and S2
   */
  private void merge(Sequence S1, Sequence S2, Comparator c, Sequence S) {

    while(!S1.isEmpty() && !S2.isEmpty()) {
      if(c.isLessThan(S1.first().element(),S2.first().element())) {
	S.insertLast(S1.removeFirst());
      }
      else {
	S.insertLast(S2.removeFirst());
      }
    }

    while(!S2.isEmpty()) {
      S.insertLast(S2.removeFirst());
      }

    while(!S1.isEmpty()) {
      S.insertLast(S1.removeFirst());
    }
  }


}