codevs 1008 选数 dfs

#include<iostream>
#include<algorithm>
#include<math.h>
using namespace std;
int a[25];
int n ,k , count_t = 0;
bool isprime(int t)
{
for(int i = 2;i <= sqrt(t)+1;i++)//第一次这里写成i <= t/2;导致一直超时~
{
if(t%i == 0) return false;
}
return true;
}
void dfs(int x,int num,int sum)
{

if(num == k && isprime(sum))
{
count_t++;
//cout << sum << endl;
}
if(num > k) return;
for(int i = x+1;i < n;i++)
{
dfs(i,num+1,sum + a[i]);
//	sum = 0;
}
}
int main()
{
cin >> n >> k;
for(int i = 0;i < n;i++)
{
cin >> a[i];
}
int sum;
for(int i = 0; i < n-k+1;i++)
{
sum = 0;
dfs(i,1,sum + a[i]);
}
cout << count_t;
return 0;
}