HDU 5753 Permutation Bo

计算每个c[i]对答案做出的贡献，累加即可。手算一下会发现是：c[1]/2+(c[2]+....c[n-1])/3+c[n]/2。注意：一个数的时候直接输出那个数。

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi = acos(-1.0), eps = 1e-8;
void File()
{
    freopen("D:\\in.txt", "r", stdin);
    freopen("D:\\out.txt", "w", stdout);
}
inline int read()
{
    char c = getchar();  while (!isdigit(c)) c = getchar();
    int x = 0;
    while (isdigit(c)) { x = x * 10 + c - '0'; c = getchar(); }
    return x;
}

int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        if(n==1) { int x; scanf("%d",&x); printf("%d\n",x); }
        else
        {
            double ans=0;
            for(int i=1;i<=n;i++)
            {
                int x; scanf("%d",&x);
                if(i==1||i==n) ans=ans+1.0*x/2;
                else ans=ans+1.0*x/3;
            }
            printf("%lf\n",ans);
        }
    }
    return 0;
}