快排

import java.util.Random;

public class Main {
Random random = new Random();

//产生随机数组用于测试
int[] generateList() {
    int N = random.nextInt(100) + 10;
    int[] a = new int[N];
    for (int i = 0; i < a.length; i++) {
        a[i] = random.nextInt(1000);
    }
    return a;
}

//判断数组a是否升序
boolean checkRight(int[] a) {
    for (int i = 1; i < a.length; i++) {
        if (a[i] < a[i - 1]) return false;
    }
    return true;
}

//随机化快排
void quickSort(int[] a, int beg, int end) {
    //递归终止条件
    if (end <= beg) return;
    //随机选择分界值，并把分界值移动到最左边
    int x = random.nextInt(end - beg) + beg;
    int sep = a[x];
    a[x] = a[beg];
    a[beg] = sep;
    //双指针交替移动，直到相遇，复杂度为O(N),无额外空间
    int l = beg, r = end - 1;
    while (true) {
        //先移动右边的指针，直到找到小于sep的值
        while (r > l && a[r] >= sep) r--;
        if (r <= l) break;
        a[l] = a[r];
        a[r] = sep;
        l++;
        //移动功能左边的指针，直到找到大于sep的值
        while (l < r && a[l] <= sep) l++;
        if (l >= r) break;
        a[r] = a[l];
        a[l] = sep;
        r--;
    }
    quickSort(a, beg, l);
    quickSort(a, l + 1, end);
}

Main() {
    for (int i = 0; i < 100; i++) {
        int[] a = generateList();
        quickSort(a, 0, a.length);
        if (!checkRight(a)) {
            System.out.println("wrong");
        }
    }
}

public static void main(String[] args) {
    new Main();
}
}