2.3.18

question:

Median-of 3 partitioning. Add median-of-3 partitioning to quicksort, as described in the text (see page 296). Run doubling tests to determine the effectivemess of the change.

answer:

//三取样切分

 //顺便用三个值中的最大值做哨兵

import edu.princeton.cs.algs4.*;

public class Quicksort
{
    public static void sort(Comparable[] a)
    {
        StdRandom.shuffle(a);
        sort(a, 0, a.length - 1);
    }
    
    private static void sort(Comparable[] a, int lo, int hi)
    {
        if(hi <= lo)
            return;
        int j = partition(a, lo, hi);
        sort(a, lo, j-1);
        sort(a, j+1, hi);
    }
    
    private static int partition(Comparable[] a, int lo, int hi)
    {
        int i = lo, j = hi+1;
        int mid = lo + (hi-lo)/2;
        int pos = MedianOfThree(a,lo,mid,hi);//返回中值的位置
        exch(a,lo,pos);//交换中值和lo处的值
        int sentinel = less(a[pos],a[hi]) ? hi : pos;//找三个值中最大的作为哨兵
        exch(a, sentinel, hi);
        Comparable v = a[lo];
        while(true)
        {
            while(less(a[++i], v));
            while(less(v,a[--j]));
            if(i >= j) break;
            exch(a, i, j);
        }
        exch(a, lo, j);
        return j;
    }
    
    public static int MedianOfThree(Comparable[] a, int lo, int mid, int hi)//返回三个位置处的中值的位置
    {
        
         return (less(a[lo], a[mid]) ?
           (less(a[mid], a[hi]) ? mid : less(a[lo], a[hi]) ? hi : lo) :
           (less(a[hi], a[mid]) ? mid : less(a[hi], a[lo]) ? hi : lo));
    }
    
    private static boolean less(Comparable v, Comparable w)
    {
        return v.compareTo(w) < 0;
    }
    
    private static void exch(Comparable[] a, int i, int j)
    {
        Comparable t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
    
    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 main(String[] args)
    {
        String[] a = In.readStrings();
        sort(a);
        assert isSorted(a);
        show(a);
    }
}