bzoj2720: [Violet 5]列队春游

传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=2720

思路：

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
const int maxn=305;
typedef long double ld;
using namespace std;
int n,h[maxn],tmp[maxn];
ld fac[maxn][maxn];

void init(){
scanf("%d",&n);
for (int i=1;i<=n;i++) scanf("%d",&h[i]),tmp[i]=h[i];
sort(tmp+1,tmp+1+n);
for (int i=1;i<=n;i++) h[i]=lower_bound(tmp+1,tmp+1+n,h[i])-tmp-1;
memset(fac,0,sizeof(fac));
fac[0][0]=1;
for (int i=1;i<=n;i++){
fac[i][0]=1;
for (int j=1;j<=i;j++)
fac[i][j]=fac[i][j-1]*(i-j+1);
}
}

void work(){
ld ans=0;
for (int i=1;i<=n;i++){//枚举小朋友
ld s=0;
for (int j=1;j<=n;j++){//枚举长度
ans+=(s+fac[h[i]][j-1]/fac[n-1][j-1]*j)/n;
s+=(n-h[i]-1)*fac[h[i]][j-1]/fac[n-1][j]*j;
}
}
printf("%.2f\n",(double)ans);
}

int main(){
init(),work();
return 0;
}