模板

• C++

//从小到大排序
void quick_sort(int q[], int l, int r)
{
    if (l >= r) return;

    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while (i < j)
    {
        do i ++ ; while (q[i] < x);
        do j -- ; while (q[j] > x);
        if (i < j) swap(q[i], q[j]);
    }
    quick_sort(q, l, j), quick_sort(q, j + 1, r);
}

//从大到小排序
void quick_sort(int q[], int l, int r)
{
    if(l >= r) return;

    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while(i < j)
    {
        do i++; while(q[i] > x); // 这里和下面
        do j--; while(q[j] < x); // 这行的判断条件改一下
        if(i < j) swap(q[i], q[j]);
    }
    quick_sort(q, l, j), quick_sort(q, j + 1, r);
}

• Java

//从小到大排序
public static void quick_sort(int[] q, int l, int r){
    if(l >= r) return;
    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while(i < j) {
        while( q[++i] < x);
        while( q[--j] > x);
        if(i < j){
            int t = q[i];
            q[i] = q[j];
            q[j] = t;
        }
    }
    quickSort(q, l, j);
    quickSort(q, j + 1, r);
}

//从大到小排序
public static void quick_sort(int[] q, int l, int r) {
    if (l >= r) {
        return;
    }
    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while (i < j) {
        while (q[++i] > x);
        while (q[++j] < x);
        if (i < j) {
            int t = q[i];
            q[i] = q[j];
            q[j] = t;
        }
    }
    quickSort(q, l, j);
    quickSort(q, j + 1, r);
}

• 以上是以j做划分，以i做划分的模板如下

void quick_sort(int q[], int l, int r)
{
    if(l >= r) return;

    int i = l - 1, j = r + 1, x = q[l + r + 1 >> 1];//注意是向上取整,因为向下取整可能使得x取到q[l]
    while(i < j)
    {
        do i++; while(q[i] < x);
        do j--; while(q[j] > x);
        if(i < j) swap(q[i], q[j]);
    }
    quick_sort(q, l, i - 1), quick_sort(q, i, r);//不用q[l..i],q[i+1..r]划分的道理和分析4中j的情况一样
}

• 详见AcWing 785. 快速排序算法的证明与边界分析 - AcWing

练习

01

• 题目

• 题解

import java.io.*;

public class Main {
    
    public static final int N = 100010;
    public static int[] q = new int[N];
    
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        String[] strs = br.readLine().split(" ");
        for (int i = 0; i < n; i++) {
            q[i] = Integer.parseInt(strs[i]);
        }
        br.close();
        quickSort(q, 0, n - 1);
        for (int i = 0; i < n; i++) {
            System.out.print(q[i] + " ");
        }
    }

    public static void quickSort(int[] q, int l, int r) {
        if (l >= r) {
            return;
        }
        int i = l - 1, j = r + 1, x = q[l + r >> 1];
        while (i < j) {
            while (q[++i] < x) ;
            while (q[--j] > x) ;
            if (i < j) {
                int t = q[i];
                q[i] = q[j];
                q[j] = t;
            }
        }
        quickSort(q, l, j);
        quickSort(q, j + 1, r);
    }
}

02

• 题目

• 题解

import java.io.*;

public class Main {
    public static final int N = 100010;
    public static int[] q = new int[N];
        
    public static void main(String args[]) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] str1 = br.readLine().split(" ");
        int n = Integer.parseInt(str1[0]);
        int k = Integer.parseInt(str1[1]);
        String[] str2 = br.readLine().split(" ");
        for (int i = 0; i < n; i++) {
            q[i] = Integer.parseInt(str2[i]);
        }
        quickSort(q, 0, n - 1);
        System.out.println(q[k - 1]);
    }

    public static void quickSort(int[] q, int l, int r) {
        if (l >= r) {
            return;
        }
        int i = l - 1, j = r + 1, x = q[l + r >> 1];
        while (i < j) {
            while (q[++i] < x) ;
            while (q[--j] > x) ;
            if (i < j) {
                int t = q[i];
                q[i] = q[j];
                q[j] = t;
            }
        }
        quickSort(q, l, j);
        quickSort(q, j + 1, r);
    }
}