函数prev

补充条件：m（arry存储）个里面选取n个。同时设置布尔数组（m个的）

以后统一设置前n个数组为1，用prev_permutation

例子：洛谷1249

#include
using namespace std;
bool isnprimes[100000000];int primes[10000000];int cnt,sum;int count1;
int k,n;
void ola(int n)
{
	for(int i=2;i<=n;i++)
	{
		if(isnprimes[i]==0)primes[++cnt]=i;
		for(int j=1;i*primes[j]<=n;j++)
		{
			isnprimes[i*primes[j]]=1;
			if(i%primes[j]==0)break;
		}
	}
}
int main()
{
	scanf("%d%d",&n,&k);
	int arry[n+1]={0};for(int i=1;i<=n;i++)scanf("%d",&arry[i]);int book[n+1]={0};
	for(int i=1;i<=k;i++)book[i]=1;
	ola(100000000);
	do
	{
		count1=0;
		for(int i=1;i<=n;i++){
		if(book[i]==1)count1+=arry[i];
	}
		if(isnprimes[count1]==0)sum++;
	}
	while(prev_permutation(book+1,book+1+n));
	printf("%d",sum);
	return 0;
}