codevs 1008 选数 dfs

题目：codevs1008

思路：就是dfs（深度优先搜索），挺简单的，第一次超时因为素数的判断放法错了~>

#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;}