6.快速排序

1.快速排序简介

 2.原理

 3.图解排序过程

　　4.代码实现

import java.util.*;
import java.io.*;

public class Quick {

    public static void main(String[] args)throws Exception {
        // Integer[] a = {10,9,8,7,6,5,4,3,2,1,0,0,-1};
        // 读取文件中的逆序数组，个人测试数据为10万
        BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(new File("arr.txt"))));
        LinkedList<Integer> list = new LinkedList<>();
        String line = "";
        while((line = br.readLine()) != null ) {
            list.add(Integer.parseInt(line));
        }
        Integer[] arr = asArray(list);
        long start = System.currentTimeMillis();
        sort(arr,0,arr.length-1);
        long end = System.currentTimeMillis();
        System.out.println("用时"+(end-start)+"毫秒");
        System.out.println(Arrays.toString(arr));
    }
    
    private static int partition(Integer[] arr, int low,int high) {
        // 对判断基准的取值使用随机数，避免碰到逆序数组的时候，时间复杂度退化成O（n^2)，而且这种情况还很容易导致栈溢出
        Random random = new Random();
        int index = low + random.nextInt(high - low);
        swap(arr,low,index);
        int mark = arr[low];
        int right = low;
        int left = high;
        
        while(true) {
            while(arr[right] <= mark) {
                if(right == high)
                    break;
                right++;
            }
            
            while(arr[left] > mark) {
                if(left == low)
                    break;
                left--;
            }
            
            if(right >= left) {
                Integer temp = arr[left];
                arr[left] = mark;
                arr[low] = temp;
                break;
            } else {
                Integer temp = arr[right];
                arr[right] = arr[left];
                arr[left] = temp;
            }
        }
        return left;
    }
    
    private static void sort(Integer[] arr,int low,int high) {
        if(low >= high)
            return;
        int mid = partition(arr,low,high);
        sort(arr,low,mid-1);
        sort(arr,mid+1,high);
    }
    
    private static Integer[] asArray(List<Integer> list) {
        Integer[] arr = new Integer[list.size()];
        for(int i = 0 ; i < list.size();i++)
            arr[i] = list.get(i);
        return arr;    
    }
    
    public static void swap(Comparable[] arr, int i, int j) {
        Comparable temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
}

5.测试结果