不剪枝

#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>

using namespace std;

const int N = 30;

int m, n;
int way[N]; 

void dfs(int u, int start) {
    if (u == m) {
        for (int i = 0; i < m; i ++ ) printf("%d ", way[i] + 1);
        puts("");
        return; 
    }

    for (int i = start; i < n; i ++ ) {
        way[u] = i;
        dfs(u + 1, i + 1);
        way[u] = 0; // 恢复现场 
    }
}

int main() {
    scanf("%d%d", &n, &m);

    dfs(0, 0);

    return 0;
}

剪枝

void dfs(int u, int start) {
    if (u + n - start < m) return;
    ...