CF449 C. Jzzhu and Apples

 /*
 http://codeforces.com/problemset/problem/449/C
 cf 449 C. Jzzhu and Apples
 数论+素数+贪心
 */
 #include <cstdio>
 #include <algorithm>
 using namespace std;
 const int Nmax=;
 int is_prime[Nmax];
 int book[Nmax];
 int cnt[Nmax];
 int n,ans;
 void get__prime()
 {
     for(int i=;i<=n;i++)
         is_prime[i]=;
     for(int i=;i<=n;i++)
     {
         if(is_prime[i])
         {
             for(int j=;j*i<=n;j++)
             {
                 is_prime[i*j]=;
             }
         }
     }
 }

 int main()
 {
     //freopen("cf449C.in","r",stdin);
     scanf("%d",&n);
     get__prime();
     for(int i=n;i>=;i--)
     {
         if(is_prime[i])
         {
             cnt[i]=;
             for(int j=;i*j<=n;j++)
             {
                 if(!book[i*j])
                 {
                     cnt[i]++;
                     book[i*j]=;
                 }
             }
             if(cnt[i]&)
                 book[*i]=;
             ans+=cnt[i]>>;
         }
     }
     printf("%d\n",ans);
     for(int i=;i<=n;i++)
         book[i]=;
     for(int i=n;i>=;i--)
     {
         if(is_prime[i] && cnt[i]>)
         {
             if(cnt[i]&)
             {
                 int flag=;
                 for(int j=;i*j<=n;j++)
                 {
                     if(j==)
                         continue;
                     if(!book[i*j])
                     {
                         printf("%d",i*j);
                         if(flag)
                             printf(" ");
                         else
                             printf("\n");
                         flag^=;
                         book[i*j]=;
                     }
                 }
             }
             else
             {
                 int flag=;
                 for(int j=;i*j<=n;j++)
                 {
                     if(!book[i*j])
                     {
                         printf("%d",i*j);
                         if(flag)
                             printf(" ");
                         else
                             printf("\n");
                         flag^=;
                         book[i*j]=;
                     }
                 }
             }
         }
     }
     return ;
 }