Java 代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
    static int n, m;
    static int[] st = new int[100];

    static void dfs(int u, int sum) {
        // 边界条件
        if (sum + n - u < m)//如果剩下的数凑不到m个
            return;
        if (sum == m) {
            for (int i = 0; i < n; i++) {
                if (st[i] == 1) {
                    System.out.print(i + 1 + " ");
                }
            }
            System.out.println();
            return;
        }

        st[u] = 1;//标志1表示选了这个数
        dfs(u + 1, sum + 1);
        st[u] = 0;//回溯，设为没选
        dfs(u + 1, sum);

    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String[] read = br.readLine().split(" ");

        n = Integer.parseInt(read[0]);
        m = Integer.parseInt(read[1]);

        dfs(0, 0);// 枚举到哪了，选了多少个

    }
}