刚学C++几天的萌新的尝试性解答

#include <iostream>
using namespace std;

int main()
{
    int n,sum,sum1,sum2;
    cin>>n;
    sum=0;sum1=1;sum2=1;
    for(int i = 1;i<=n;i++){
        if(i<=sum1){
            sum += sum2;
        }
        else{
            sum2 +=1;
            sum1 +=sum2;
            sum += sum2;
        }
    }
    cout<<sum;
  	return 0;
}