代码很相近，只需稍微改动即可在排列与组合中切换

组合

import java.io.IOException;
import java.util.Scanner;

class MC {

    int N = 10;
    int[] a = new int[N];
    boolean[] st = new boolean[N];
    //C(k1, k2)
    int k1 = 9, k2 = 2;

    void check() {
        for (int i = 1; i <= k2 ; i++) {
            System.out.print(a[i] + " ");
        }
        System.out.println();
    }

    void dfs(int cnt) {
        if (cnt == k2 + 1){
            check();
            return;
        }
        for (int i = 1; i <= k1; i++) {
            if (st[i] ||  i < a[cnt - 1]) continue;
            st[i] = true;
            a[cnt] = i;
            dfs(cnt + 1);
            st[i] = false;
        }
    }

    public void run() throws IOException {
        dfs(1);
    }

    Scanner sc = new Scanner(System.in);
}

public class Main {
    public static void main(String[] args) throws IOException {
        new MC().run();
    }
}

排列

改动点：
1. 让k1与k2相等
2. 去掉i < a[cnt - 1]

import java.io.IOException;
import java.util.Scanner;

class MC {

    int N = 10;
    int[] a = new int[N];
    boolean[] st = new boolean[N];
    //A(k1, k2)
    int k1 = 3, k2 = 3;

    void check() {
        for (int i = 1; i <= k2 ; i++) {
            System.out.print(a[i] + " ");
        }
        System.out.println();
    }

    void dfs(int cnt) {
        if (cnt == k2 + 1){
            check();
            return;
        }
        for (int i = 1; i <= k1; i++) {
            if (st[i]) continue;
            st[i] = true;
            a[cnt] = i;
            dfs(cnt + 1);
            st[i] = false;
        }
    }

    public void run() throws IOException {
        dfs(1);
    }

    Scanner sc = new Scanner(System.in);
}

public class Main {
    public static void main(String[] args) throws IOException {
        new MC().run();
    }
}