Atcoder Code Festival 2017 Team Relay J - Indifferent

Posted 2020-12-02 memory-of-winter

题目大意：共$2n$个价格$p_i$。两人轮流取。你每次取最大的，对方每次随机取。问你取的期望和是多少。

题解：从小到大排序，$sumlimits_{i=0}^{2n-1} frac{i*p_i}{2n-1}$

卡点：无

 

C++ Code：

#include<cstdio>
#include<algorithm>
using namespace std;
long long n,s[200005];
long long ans;
int main(){
	scanf("%lld",&n);
	for (int i=0;i<n*2;i++)scanf("%lld",&s[i]);
	sort(s,s+n*2);
	for (long long i=0;i<n*2;i++)ans+=i*s[i];
	printf("%.10lf",ans/(double)(2*n-1));
	return 0;
}