2.2.11

question:

Improvements. Implement the three improvements to merge sort that are described in the text on page 275: Add a cutoff for small subarrays, test whether the array is already in order, and avoid the copy by switching arguments in the recurisive code.

answer:

//对归并排序进行三处优化

import edu.princeton.cs.algs4.*;

public class Merge
{   
    public static void sort(Comparable[] a)
    {
        Comparable[] b = new Comparable[a.length];
        sort(a, b, 0, a.length-1);
    }
    
    private static boolean less(Comparable v, Comparable w)
    {
        return v.compareTo(w) < 0;
    }
    
    private static void show(Comparable[] a)
    {
        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 merge(Comparable[] a, Comparable[] b, int lo, int mid, int hi)//传两个数组用于对掉原来的a和aux
{
    //这里直接把a[]归并到b[]里面,然后把b当成a
    int i = lo, j = mid + 1;
    
    for(int k = lo; k <= hi; k++)//其实这里还可以用上题2.2.10来优化,这题没要求我就没改了,可参见上题
    {
        if(i > mid)
            b[k] = a[j++];
        else if(j > hi)
            b[k] = a[i++];
        else if(less(a[i], a[j]))
            b[k] = a[i++];
        else
            b[k] = a[j++];
    }
}
    
    private static void sort(Comparable[] a, Comparable[] b, int lo, int hi)//传两个数组用于对掉原来的a和aux
    {
        //优化3,主要就是这里sort,和merge参数的互换减少了原来归并里复制a到aux里
        if(a.length <= 15)//优化1,数组过小则用插入排序
        {
            Selection.sort(a);
            return;
        }
        if(hi <= lo)
            return;
        int mid = lo + (hi - lo) / 2;
        sort(b,a,lo,mid);//这里就是把b当成a
        sort(b,a,mid+1,hi);//这里也是把b当成a
        if(!(a[mid].compareTo(a[mid+1])<0))//优化2,测试数组是否已经有序
            merge(a,b,lo,mid,hi);//归并a
    }
    
    public static void main(String[] args)
    {
        String[] a = In.readStrings();
        sort(a);
        assert isSorted(a);
        show(a);
    }
}