洛谷 - P3935 - Calculating - 整除分块

https://www.luogu.org/fe/problem/P3935

求：

\(F(n)=\sum\limits_{i=1}^{n}d(i)\)

枚举因子\(d\)，每个因子\(d\)都给其倍数贡献\(1\)，倍数一共有\(\lfloor\frac{n}{d}\rfloor\)个。

\(F(n)=\sum\limits_{d=1}^{n}\lfloor\frac{n}{d}\rfloor\)

套个分块，上。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

const int mod=998244353;

ll F(ll n){
    ll res=0;
    for(ll l=1,r;l<=n;l=r+1){
        ll t=n/l;
        r=n/t;
        res+=t*(r-l+1);
        if(res>=mod)
            res%=mod;
    }
    return res;
}

int main() {
#ifdef Yinku
    freopen("Yinku.in","r",stdin);
#endif // Yinku
    ll l,r;
    scanf("%lld%lld\n",&l,&r);
    printf("%lld\n",(F(r)-F(l-1)+mod)%mod);
    return 0;
}