NOIP2002普及组复赛B 选数

题目链接：https://ac.nowcoder.com/acm/contest/230/B

题目大意：

　　略

分析：

　　DFS模板题。

代码如下：

 #include <bits/stdc++.h>
 using namespace std;

 #define rep(i,n) for (int i = 0; i < (n); ++i)
 #define For(i,s,t) for (int i = (s); i <= (t); ++i)
 #define rFor(i,t,s) for (int i = (t); i >= (s); --i)
 #define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i)
 #define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i)

 #define pr(x) cout << #x << " = " << x << "  "
 #define prln(x) cout << #x << " = " << x << endl

 #define ALL(x) x.begin(),x.end()
 #define INS(x) inserter(x,x.begin())

 #define ms0(a) memset(a,0,sizeof(a))
 #define msI(a) memset(a,inf,sizeof(a))

 #define pii pair<int,int>
 #define piii pair<pair<int,int>,int>
 #define mp make_pair
 #define pb push_back
 #define fi first
 #define se second

 inline int gc(){
     static const int BUF = 1e7;
     static char buf[BUF], *bg = buf + BUF, *ed = bg;

     if(bg == ed) fread(bg = buf, , BUF, stdin);
     return *bg++;
 }

 inline int ri(){
     int x = , f = , c = gc();
     for(; c<||c>; f = c=='-'?-:f, c=gc());
     for(; c>&&c<; x = x* + c - , c=gc());
     return x*f;
 }

 typedef long long LL;
 const int maxN = 1e5 + ;

 int n, k, ans;
 int x[];

 bool isPrime(int x) {
     assert(x > );
     For(i, , (int)(sqrt(x) + 1e-)){
         if(x % i == ) return false;
     }
     return true;
 }

 void dfs(int a, int cnt, int sum) {
     if(cnt == k) {
         if(isPrime(sum)) ++ans;
         return;
     }
     if(a > n) return;

     dfs(a+, cnt+, sum+x[a]);
     dfs(a+, cnt, sum);
 }

 int main(){
     scanf("%d%d", &n, &k);
     For(i, , n) x[i] = ri();

     dfs(, , );

     printf("%d\n", ans);
     return ;
 }