BZOJ 1257 [CQOI2007]余数之和sum ——Dirichlet积

【题目分析】

卷积很好玩啊。

【代码】

#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>

#include <map>
#include <set>
#include <queue>
#include <string>
#include <iostream>
#include <algorithm>

using namespace std;

#define maxn 500005
#define ll long long
#define inf 0x3f3f3f3f
#define F(i,j,k) for (int i=j;i<=k;++i)
#define D(i,j,k) for (int i=j;i>=k;--i)

void Finout()
{
    #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
    freopen("wa.txt","w",stdout);
    #endif
}

int Getint()
{
    int x=0,f=1; char ch=getchar();
    while (ch<'0'||ch>'9') {if (ch=='-') f=-1; ch=getchar();}
    while (ch>='0'&&ch<='9') {x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}

int n,k,r,l;
ll ans=0;

int main()
{
    Finout();
    n=Getint(); k=Getint();
    ans=(ll)n*(ll)k;
//    cout<<"ans  is "<<ans<<endl;
	int now=1; r=min(n,k);
    while (r)
    {
    	now=k/r;
//    	cout<<now<<" "<<n<<" "<<k<<" "<<r<<endl;
    	l=k/(now+1)+1;
//    	cout<<"now "<<now<<endl;
//    	cout<<l<<"---"<<r<<endl;
    	ans-=((ll)r+(ll)l)*((ll)r-(ll)l+1)/2LL*(ll)now;
//    	cout<<"ans is "<<ans<<endl;
    	r=l-1;
//    	cout<<"r is "<<r<<endl;
	}
	cout<<ans<<endl;
}