Given a sequence of positive numbers, a segment is defined to be a consecutive subsequence. For example, given the sequence {0.1, 0.2, 0.3, 0.4}, we have 10 segments: (0.1) (0.1, 0.2) (0.1, 0.2, 0.3) (0.1, 0.2, 0.3, 0.4) (0.2) (0.2, 0.3) (0.2, 0.3, 0.4) (0.3) (0.3, 0.4) (0.4).

Now given a sequence, you are supposed to find the sum of all the numbers in all the segments. For the previous example, the sum of all the 10 segments is 0.1 + 0.3 + 0.6 + 1.0 + 0.2 + 0.5 + 0.9 + 0.3 + 0.7 + 0.4 = 5.0.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N, the size of the sequence which is no more than 105. The next line contains N positive numbers in the sequence, each no more than 1.0, separated by a space.

Output Specification:

For each test case, print in one line the sum of all the numbers in all the segments, accurate up to 2 decimal places.

Sample Input:

4
0.1 0.2 0.3 0.4

Sample Output:

5.00
#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
int main(){
int N;
double ans = , temp;
scanf("%d", &N);
for(int i = ; i <= N; i++){
scanf("%lf", &temp);
ans += 1.0 * i * (N + - i) * temp;
}
printf("%.2lf", ans);
cin >> N;
return ;
}

总结:

1、主要是找规律, 计算出每个数一共出现几次(与位置有关),然后做乘法再累加即可。第i个数(从1开始)出现的次数是:i * (n + 1 - i)。

2、i * (N + 1 - i)有可能超过int的范围。 所以做类型转换时,1.0应该乘在式子的最前面。否则当int溢出后再做转换就无效了。

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