算法（Algorithms）第4版 练习 2.2.11（3）

关键代码实现：

　　public static void sort(Comparable[] input) {
        int N = input.length;
        aux = input.clone();//must be clone(the same as input)
//        StdOut.println("input:" + input + " aux:" + aux);//for test
        sort(aux, input, 0, N-1);
    }

    //take input from source, sort it and put output in destination
    //source and destination is the same at the beginning
    private static void sort(Comparable[] source, Comparable[] destination, int lo, int hi) {

        if(lo >= hi)//just one entry in array
            return;

        int mid = lo + (hi-lo)/2;
        sort(destination, source, lo, mid);//first get input from destination, sort it and put output in source
        sort(destination, source, mid+1, hi);
        merge(source, destination, lo, mid, hi);//merge sorted source to destination

    }

    private static void merge(Comparable[] source, Comparable[] destination, int lo, int mid, int hi) {

        int i = lo;
        int j = mid + 1;
        for(int k = lo; k <= hi; k++) {
            if(i > mid)
                destination[k] = source[j++];
            else if(j > hi)
                destination[k] = source[i++];
            else if(less(source[j], source[i]))
                destination[k] = source[j++];
            else
                destination[k] = source[i++];
        }
//        StdOut.println("source:" + source + " destination:" + destination);
//        StdOut.printf("merge(source, destination, %4d, %4d, %4d)", lo, mid, hi);
//        show(destination);//for test

    }

测试用例：

package com.qiusongde;

import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.StdOut;

public class MergeAvoidCopy {

    private static Comparable[] aux;

    public static void sort(Comparable[] input) {
        int N = input.length;
        aux = input.clone();//must be clone(the same as input)
//        StdOut.println("input:" + input + " aux:" + aux);//for test
        sort(aux, input, 0, N-1);
    }

    //take input from source, sort it and put output in destination
    //source and destination is the same in start
    private static void sort(Comparable[] source, Comparable[] destination, int lo, int hi) {

        if(lo >= hi)//just one entry in array
            return;

        int mid = lo + (hi-lo)/2;
        sort(destination, source, lo, mid);//first get input from destination, sort it and put output in source
        sort(destination, source, mid+1, hi);
        merge(source, destination, lo, mid, hi);//merge sorted source to destination

    }

    private static void merge(Comparable[] source, Comparable[] destination, int lo, int mid, int hi) {

        int i = lo;
        int j = mid + 1;
        for(int k = lo; k <= hi; k++) {
            if(i > mid)
                destination[k] = source[j++];
            else if(j > hi)
                destination[k] = source[i++];
            else if(less(source[j], source[i]))
                destination[k] = source[j++];
            else
                destination[k] = source[i++];
        }
//        StdOut.println("source:" + source + " destination:" + destination);
//        StdOut.printf("merge(source, destination, %4d, %4d, %4d)", lo, mid, hi);
//        show(destination);//for test

    }

    private static boolean less(Comparable v, Comparable w) {

        return v.compareTo(w) < 0;

    }

    private static void show(Comparable[] a) {

        //print the array, on a single line.
        for(int i = 0; i < a.length; i++) {
            StdOut.print(a[i] + " ");
        }
        StdOut.println();

    }

    public static boolean isSorted(Comparable[] a) {

        for(int i = 1; i < a.length; i++) {
            if(less(a[i], a[i-1]))
                return false;
        }

        return true;

    }

    public static void main(String[] args) {

        //Read strings from standard input, sort them, and print.
        String[] input = In.readStrings();
//        show(input);//for test
        sort(input);
        assert isSorted(input);
//        show(input);//for test

    }

}

测试结果：

M E R G E S O R T E X A M P L E
input:[Ljava.lang.String;@1b6d3586 aux:[Ljava.lang.String;@4554617c
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    0,    0,    1)E M R G E S O R T E X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    2,    2,    3)E M G R E S O R T E X A M P L E
source:[Ljava.lang.String;@4554617c destination:[Ljava.lang.String;@1b6d3586
merge(source, destination,    0,    1,    3)E G M R E S O R T E X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    4,    4,    5)E M G R E S O R T E X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    6,    6,    7)E M G R E S O R T E X A M P L E
source:[Ljava.lang.String;@4554617c destination:[Ljava.lang.String;@1b6d3586
merge(source, destination,    4,    5,    7)E G M R E O R S T E X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    0,    3,    7)E E G M O R R S T E X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    8,    8,    9)E E G M O R R S E T X A M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,   10,   10,   11)E E G M O R R S E T A X M P L E
source:[Ljava.lang.String;@4554617c destination:[Ljava.lang.String;@1b6d3586
merge(source, destination,    8,    9,   11)E G M R E O R S A E T X M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,   12,   12,   13)E E G M O R R S E T A X M P L E
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,   14,   14,   15)E E G M O R R S E T A X M P E L
source:[Ljava.lang.String;@4554617c destination:[Ljava.lang.String;@1b6d3586
merge(source, destination,   12,   13,   15)E G M R E O R S A E T X E L M P
source:[Ljava.lang.String;@1b6d3586 destination:[Ljava.lang.String;@4554617c
merge(source, destination,    8,   11,   15)E E G M O R R S A E E L M P T X
source:[Ljava.lang.String;@4554617c destination:[Ljava.lang.String;@1b6d3586
merge(source, destination,    0,    7,   15)A E E E E G L M M O P R R S T X
A E E E E G L M M O P R R S T X 

性能比较：

For 20000 random Doubles 1000 trials
Merge is 3.4s
MergeFasterM is 3.1s
MergeUseInsert is 3.2s
MergeSkipMerge is 3.3s
MergeAvoidCopy is 3.0s