HDOJ - 4503 湫湫系列故事——植树节

    这道题就是抄袭Codeforces的一道原题..http://blog.csdn.net/kk303/article/details/8043469..把三人都是朋友看成Alice..三人都不是朋友看成Bob..一个题... 

Program:

#include<iostream>#include<stdio.h>#include<string.h>#include<cmath>#include<algorithm>#define ll long longusing namespace std;  int n;int main(){       //  freopen("input.txt","r",stdin);   freopen("output.txt","w",stdout);       int T,x,i,sum,m;      scanf("%d",&T);      while (T--)      {            scanf("%d",&n);            sum=n*(n-1)*(n-2)/6;            m=0;            for (i=1;i<=n;i++)            {                 scanf("%d",&x);                 m+=x*(n-1-x);            }            m/=2;            m=sum-m;            printf("%.3lf\n",m*1.0/sum);      }      return 0;}