代码

#include<bits/stdc++.h>
using namespace std;
int n=5e6,a[5000010];
int main()
{
	for(int i=1; i<=n; ++i) a[i]=i;
	long long sum=0;
	for(int i=1; i<=n; ++i)
	{
		int el=1,er=n,ans;
		while(el<=er)
		{
			int mid=el+er>>1;
			if(a[mid]>=i) ans=mid,er=mid-1;
			else el=mid+1;
		}
		sum+=ans;
	}
	cout<<sum;
	return 0;
}

评测结果