质数中的质数（质数筛法）

 1 #include<cstdio>
 2 const int maxn=1100000;
 3 const int minn=10000;
 4 int n,a,b;
 5 int s[maxn/10],m;
 6 bool v[maxn];
 7 int main(){
 8     scanf("%d",&n);
 9     v[0]=v[1]=1;
10     for(int i=2;i<=n+minn;i++)
11     if(!v[i]){
12         a=i+i;s[m++]=i;
13         while(a<=n+minn){v[a]=1;a+=i;}
14     }
15     while(s[b]<n||v[b+1]) b++;
16     printf("%d\n",s[b]);
17     return 0;
18 }