UVA-11426

 https://vjudge.net/problem/UVA-11426

参考这个博客

https://www.cnblogs.com/war1111/p/7641546.html

修改的地方为：

(n/k-1)*(n/k)/2

#include <bits/stdc++.h>
#define inf 2333333333333333
#define N 4000010
#define p(a) putchar(a)
#define For(i,a,b) for(long long i=a;i<=b;++i)
//by war
//2021.1.29
using namespace std;
long long n;
long long f[N];
long long ans;

void in(long long &x){
    long long y=1;char c=getchar();x=0;
    while(c<'0'||c>'9'){if(c=='-')y=-1;c=getchar();}
    while(c<='9'&&c>='0'){ x=(x<<1)+(x<<3)+c-'0';c=getchar();}
    x*=y;
}
void o(long long x){
    if(x<0){p('-');x=-x;}
    if(x>9)o(x/10);
    p(x%10+'0');
}

signed main(){
    while(cin>>n && n!=0){
        ans=0;
        for(long long k=n;k>=1;k--){
            long long sum=0;
            For(i,2,n){
                if(i*k<=n) sum+=f[i*k];
                else break;
            }
            f[k]=(n/k-1)*(n/k)/2-sum;
        }
        for(long long k=n;k>=1;k--) ans+=k*f[k];
        o(ans);p('
');
    }
    return 0;
}