筛选法求素数

const int maxn = 1000010;  bool vis[maxn];  int prime[maxn];    int sieve(int n)  {      memset(vis, 0, sizeof(vis));      vis[0] = vis[1] = 1;      int c = 0;      for(int i = 2; i <= n; i++)          if(!vis[i])          {              prime[c++] = i;              for(int j = 2*i; j <= n; j += i)                  vis[j] = 1;          }      return c;  }