快速排序及其优化

目录

package leetcode.mySort;

import java.util.Random;

public class QuickSort {

    private final static Random random = new Random(System.currentTimeMillis());

//    快速排序的不同类型的写法，差别在于partition  下面的partition是大学时候老师教的方法  partition2是
//    类似三色国旗的写法，为后续的优化做准备 partition和partition2本质上都是二路快排

    public static int partition(int[] array,int start,int end){
        int base = array[start];
        while (start<end){
            while (start<end&&array[end]>=base)  end--;
            array[start] = array[end];

            while (start<end&&array[start]<=base)  start++;
            array[end] = array[start];
        }
        array[start] = base;
        return start;
    }

    public static int partition2(int[] array,int start,int end){
        int pivot = array[start];

        // j代表较小区间的最后一个
        // all in array[left+1,j] <= pivot
        // all in array(j,i) > pivot

        int j = start;

        // 大方过，小交换
        for (int i = start+1; i <= end; i++) {
            if (array[i]<pivot){
                j++;
                swap(array,i,j);
            }
        }
        swap(array,start,j);
        return j;
    }

//    随机初始化pivot
    public static int partition3(int[] array,int start,int end){
        int randomIndex =start + random.nextInt(end - start + 1);
        swap(array,start,randomIndex);
        int pivot = array[start];
        int j = start;

        for (int i = start+1; i <= end; i++) {
            if(pivot>array[i]){
                j++;
                swap(array,j,i);
            }

        }
        swap(array,start,j);
        return j;
    }

//    三路快排
    public static void quickSort3(int[] array, int start, int end){

        if (start>=end){
            return;
        }

        int randomIndex = start + random.nextInt(end - start + 1);
        swap(array,start,randomIndex);
        int pivot = array[start];

        int lt = start + 1;     // lt:less than
        int gt = end;           // gt:greater than

        // all in nums[left+1,lt) < pivot
        // all in nums[lt,i) = pivot
        // all in nums(gt,right] > pivot
        int i = start + 1;

        while (i <= gt){
            if(array[i]<pivot){
                swap(array,i,lt);
                i++;
                lt++;
            }else if (array[i]==pivot){
                i++;
            }else{
                swap(array,i,gt);
                gt--;
            }
        }
        swap(array,start,lt-1);
        quickSort3(array,start,lt-1);
        quickSort3(array,gt+1,end);
    }


    private static void swap(int[] array, int start, int end) {
        int temp = array[start];
        array[start] = array[end];
        array[end] = temp;
    }

    public static void quickSort(int[] array,int start,int end){

        if (start<end){
            int index = partition3(array,start,end);
            quickSort(array,start,index-1);
            quickSort(array,index+1,end);
        }
    }


    public static void main(String[] args) {
//        int[] array = {2,4,6,1,3,5};
        int[] array = {3,2,3,1,2,4,5,5,6};
        quickSort3(array,0,array.length-1);
        for (int i = 0; i < array.length; i++) {
            System.out.println(array[i]);
        }
    }
}