题目
实际上一开始看到题目我是打算写筛法的,后来发先是个除法分块秒切题目
显然
$$
\sum_{i = 1}^{n} f(i) = \sum_{i = 1}^{n} \lfloor \frac{i}{n} \rfloor
$$
除法分块即可
代码
#include <cstdio>
int n;
long long ans;
int main(){
scanf("%d", &n);
for(int left = 1, rig; left <= n; left = rig + 1){
rig = n / (n / left);
ans += 1LL * (n / left) * (rig - left + 1);
}
printf("%lld", ans);
}