algorithm诚不欺我

$next\_permutation$才是永远的神！

#include<iostream>
#include<algorithm>
using namespace std;
int n,r;
int a[25];
int main(){
    cin>>n>>r;
    for(int i=r+1;i<=n;i++) a[i]=1;
    do{
        for(int i=1;i<=n;i++) if(a[i]==0) cout<<i<<' ';
        cout<<endl;
    }while(next_permutation(a+1,a+1+n));
    return 0;
}