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, 1, BUF, stdin);
    return *bg++;
}
 
inline int ri(){
    int x = 0, f = 1, c = gc();
    for(; c<48||c>57; f = c=='-'?-1:f, c=gc());
    for(; c>47&&c<58; x = x*10 + c - 48, c=gc());
    return x*f;
}
 
typedef long long LL;
const int maxN = 1e5 + 7;
 
int n, k, ans;
int x[21];
 
bool isPrime(int x) {
    assert(x > 1);
    For(i, 2, (int)(sqrt(x) + 1e-6)){
        if(x % i == 0) 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+1, cnt+1, sum+x[a]);
    dfs(a+1, cnt, sum);
}
 
int main(){
    scanf("%d%d", &n, &k);
    For(i, 1, n) x[i] = ri();
     
    dfs(1, 0, 0);
     
    printf("%d
", ans);
    return 0;
}