例如 n=5,r=3，所有组合为：

123,124,125,134,135,145,234,235,245,345。

#include <iostream>
//#include <bits/stdc++.h>
#include <string>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <cstdio>
#include <set>
#include <map>
#include <vector>
#include <deque>
#include <iomanip>
using namespace std;
typedef long long ll;
//const int N = 2e5 + 10;
//int a[N], b[N];
//int aa[200], bb[200];
int arr[30];
bool a[30];
int n, r;
//int ok;
void dfs(int x, int ok)
{
    if (x > r)
    {
        for (int i = 1; i <= r; i++)
        {
            cout << setw(3) << arr[i];
        }
        cout << endl;
        return;
    }
    for (int i = ok; i <= n; i++)
    {
        if (a[i] != true)
        {
            arr[x] = i;
            a[i] = true;
            dfs(x + 1, i + 1);
            arr[x] = 0;
            a[i] = false;
        }
    }
}
int main()
{
    cin >> n >> r;
    dfs(1, 1);
    return 0;
}