BZOJ1257: [CQOI2007]余数之和sum

题目：http://www.lydsy.com/JudgeOnline/problem.php?id=1257

n%i=n-n/i*x 然后我们就可以枚举这个n/i，发现这是一个等差数列

#include<cstring>
#include<iostream>
#include<algorithm>
#include<cstdio>
#define maxn 109
#define rep(i,l,r) for (int i=l;i<=r;i++)
#define down(i,l,r) for (int i=l;i>=r;i--)
#define clr(x,y) memset(x,y,sizeof(x))
#define ll long long
#define inf int(1e9)
using namespace std;
int k,n,r,l,t;
ll ans;
int read(){
    int x=0,f=1; char ch=getchar();
    while (!isdigit(ch)) {
        if (ch=='-') f=-1; ch=getchar();
    }
    while (isdigit(ch)){
        x=x*10+ch-'0'; ch=getchar();
    }
    return x*f;
}
int main(){
    k=read(); n=read();
    if (k>n) {
        ans+=(ll)(k-n)*n; k=n;
    }
    for (int i=1;i<=k;i=r+1){
        t=n/i; r=min(k,n/t); l=i;
        ans+=(ll)(r-l+1)*n-(ll)(r-l+1)*(r+l)*t/2;
    }
    printf("%lld\n",ans);
    return 0;
}