算法（Algorithms）第4版 练习 2.2.26

在sort函数创建aux数组：

package com.qiusongde;

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

public class MergeNoStaticArray {

    public static void sort(Comparable[] input) {
        int N = input.length;
        Comparable[] aux = new Comparable[N];
        sort(input, aux, 0, N-1);
    }

    private static void sort(Comparable[] input, Comparable[] aux, int lo, int hi) {

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

        int mid = lo + (hi-lo)/2;
        sort(input, aux, lo, mid);
        sort(input, aux, mid+1, hi);
        merge(input, aux, lo, mid, hi);

    }

    private static void merge(Comparable[] input, Comparable[] aux, int lo, int mid, int hi) {

        //copy input[lo,hi] to aux[lo,hi]
        for(int i = lo; i <= hi; i++) {
            aux[i] = input[i];
        }

        int i = lo;
        int j = mid + 1;
        for(int k = lo; k <= hi; k++) {
            if(i > mid)
                input[k] = aux[j++];
            else if(j > hi)
                input[k] = aux[i++];
            else if(less(aux[j], aux[i]))
                input[k] = aux[j++];
            else
                input[k] = aux[i++];
        }

        StdOut.printf("merge(input, %4d, %4d, %4d)", lo, mid, hi);
        show(input);//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

    }

}

在merge函数创建aux数组：

package com.qiusongde;

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

public class MergeArrayCreation {

    public static void sort(Comparable[] input) {
        int N = input.length;
        sort(input, 0, N-1);
    }

    private static void sort(Comparable[] input, int lo, int hi) {

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

        int mid = lo + (hi-lo)/2;
        sort(input, lo, mid);
        sort(input, mid+1, hi);
        merge(input, lo, mid, hi);

    }

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

        //copy input[lo,hi] to aux[lo,hi]
        Comparable[] aux = new Comparable[hi-lo+1];
        int auxmid = mid - lo;
        int auxhi = hi - lo;
        for(int i = lo; i <= hi; i++) {
            aux[i - lo] = input[i];
        }

        int i = 0;//lo in aux array
        int j = (auxmid) + 1;//mid + 1 in aux array
        for(int k = lo; k <= hi; k++) {
            if(i > auxmid)
                input[k] = aux[j++];
            else if(j > auxhi)
                input[k] = aux[i++];
            else if(less(aux[j], aux[i]))
                input[k] = aux[j++];
            else
                input[k] = aux[i++];
        }

        StdOut.printf("merge(input, %4d, %4d, %4d)", lo, mid, hi);
        show(input);//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

    }

}

性能对比：

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Exercise2226 {

    public static void main(String[] args) {
        String alg1 = "MergeNoStaticArray";
        String alg2 = "MergeArrayCreation";
        int N = 20000;
        int T = 1000;

        double t1 = SortCompare.timeRandomInput(alg1, N, T);
        double t2 = SortCompare.timeRandomInput(alg2, N, T);

        StdOut.printf("For %d random Doubles %d trials\n", N, T, alg1);
        StdOut.printf("%s is %.1fs %s is %.1fs", alg1, t1, alg2, t2);
    }

}

For 20000 random Doubles 1000 trials
MergeNoStaticArray is 3.3s MergeArrayCreation is 3.9s