// 超时啦！
//
#include "stdafx.h"
#include<iostream>
#include<vector>
#include<numeric>
#include<iomanip> //cout格式化输出！
using namespace std;
int main()
{
unsigned n;
cin >> n;
double tmp;
vector<double> data;
for (unsigned i = 0; i < n; ++i)
{
cin >> tmp;
data.push_back(tmp);
}
double sum = 0.0;
for (size_t i = 0; i < n; ++i)
{
vector<double> tmp_data;
tmp_data.push_back(data[i]);
sum =sum+ accumulate(tmp_data.begin(), tmp_data.end(),0.0); //第三个实参决定了容器的类型。所累加对象必须与第三个参数类型一直！
for (size_t j = i + 1; j < n; ++j)
{
tmp_data.push_back(data[j]);
sum = sum+accumulate(tmp_data.begin(), tmp_data.end(), 0.0);
}
}
cout.setf(ios::fixed);
cout << setprecision(2) << sum;
system("pause");
return 0;
}
// 正确姿势！每个元素累加的次数存在规律！
//
#include "stdafx.h"
#include<iostream>
#include<vector>
#include<numeric>
#include<iomanip> //cout格式化输出！
using namespace std;
int main()
{
unsigned n;
cin >> n;
double tmp;
vector<double> data;
for (unsigned i = 0; i < n; ++i)
{
cin >> tmp;
data.push_back(tmp);
}
double sum = 0.0;
for (size_t i = 1, j = n; i <= n; ++i, --j)
{
sum = sum + i*j*data[i-1]; //（第i-1号元素，累加的次数为i*j！）
}
cout.setf(ios::fixed);
cout << setprecision(2) << sum;
system("pause");
return 0;
}
