[codeforces] 449C Jzzhu and Apples

原题

质因数分解后贪心即可（最后贪2）

#include<cstdio>
#include<vector>
#include<stack>
#include<vector>
#include<cstring>
#define N 100010
using namespace std;
bool is[N],mk[N];
int prime[N],num=1,cnt,n,a,b;
stack <int> ans;
vector <pair <int,int> > v;

int main()
{
    memset(is,1,sizeof(is));
    for (int i=2;i<=100000;i++)
    {
	if (is[i]) prime[++cnt]=i;
	for (int j=1;j<=cnt && i*prime[j]<=100000;j++)
	{
	    is[i*prime[j]]=0;
	    if (i%prime[j]==0) break;
	}
    }
    scanf("%d",&n);
    for (int i=2;i<=cnt && prime[i]<=n;i++)
    {
	num=1;
	while(!ans.empty()) ans.pop();
	ans.push(prime[i]);
	mk[prime[i]]=1;
	for (int j=3*prime[i];j<=n;j+=prime[i])
	{
	    if (mk[j]) continue;
	    mk[j]=1;
	    ans.push(j);
	    num++;
	}
	if (num&1 && 2*prime[i]<=n && !mk[2*prime[i]])
	{
	    mk[2*prime[i]]=1;
	    ans.push(2*prime[i]);
	}
	while (ans.size()>1)
	{
	    a=ans.top();
	    ans.pop();
	    b=ans.top();
	    ans.pop();
	    v.push_back(make_pair(a,b));
	}
    }
    while(!ans.empty()) ans.pop();
    num=0;
    for (int i=2;i<=n;i+=2)
    {
	if (mk[i]) continue;
	ans.push(i);
	mk[i]=1;
    }
    while (ans.size()>1)
    {
	a=ans.top();
	ans.pop();
	b=ans.top();
	ans.pop();
	v.push_back(make_pair(a,b));
    }
    int s=v.size();
    printf("%d\n",s);
    for (int i=0;i<s;i++)
	printf("%d %d\n",v[i].first,v[i].second);
    return 0;
}