乱选枚举+最后检查

#include <bits/stdc++.h>
using namespace std;
int n, r;
int ans[10]; // ans[i] 记录第 i 层选了谁
void dfs(int now)
{
    if (now > r)
    {
        bool flag = true;
        for (int i = 2; i <= r; i++)
            if (ans[i - 1] >= ans[i])
                flag = false;
        if (flag)
        {
            for (int i = 1; i <= r; i++)
                cout << "  " << ans[i];
            cout << "\n";
        }
        return;
    }
    for (int i = 1; i <= n; i++)
    {
        ans[now] = i;
        dfs(now + 1);
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> r;
    dfs(1);
    return 0;
}

保证每一步都合法

#include <bits/stdc++.h>
using namespace std;
int n, r;
int ans[10]; // ans[i] 记录第 i 层选了谁
void dfs(int now)
{
    if (now > r)
    {
        for (int i = 1; i <= r; i++)
            cout << "  " << ans[i];
        cout << "\n";
        return;
    }
    for (int i = ans[now - 1] + 1; i <= n; i++)
    {
        ans[now] = i;
        dfs(now + 1);
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> r;
    dfs(1);
    return 0;
}

参数传状态（TLE/MLE/RE 警告）

#include <bits/stdc++.h>
using namespace std;
int n, r;
void dfs(int now, int pre, string s)
{
    if (now > r)
    {
        cout << s << "\n";
        return;
    }
    for (int i = pre + 1; i <= n; i++)
        dfs(now + 1, i, s + "  " + (char)(i + '0'));
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> r;
    dfs(1, 0, "");
    return 0;
}