真 题解

#include <bits/stdc++.h>
using namespace std;
int a[100];
int b[50];
int vis[100];
int n,k,ans;
void dfs(int x,int sum)
{
    if(x == k+1)
    {
        bool h=1;
        for(int i = 2;i*i <= sum;i++)
        {
            if(sum % i == 0)
            {
                h = 0;   
            }
        }
        if(h)
        {
    	    ans++;
		}
	return;
    }   
	for(int i = a[x-1]+1;i <= n;i++)
	{
        if(vis[i]==1) continue;
		a[x] = i;
        vis[i]=1;
        sum += b[i];
		dfs(x+1,sum);
		sum -= b[i];//回溯
        vis[i]=0;//回溯
	}
}
int main()
{
	cin >> n >> k;
	for(int i = 1;i <= n;i++)
	{
		cin >> b[i];
	}
	dfs(1,0);
	cout << ans;
 }