OJ-Problems-Source/HDOJ/5425_autoAC.cpp
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5400-5504(END by now)
2016-09-13 04:55:33 +08:00

54 lines
1.2 KiB
C++

#include<iostream>
#include<cstring>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<cmath>
#include<queue>
using namespace std;
#define MAXN 100005
vector<int>vec[MAXN];
queue<int>q;
int n,d[MAXN];
inline double f(double a,double b)
{
return a * b / (a + b);
}
double solve()
{
double ans = 0;
for(int i = n - 1;i >= max(n - 100,0);i--)
for(int j = n - 1;j > i;j--)
ans += f(d[i],d[j]) / (pow(2.0,n - i) - (n + 1) / pow(2.0,i));
return ans;
}
int main()
{
while(scanf("%d",&n) != EOF)
{
for(int i = 0;i <= n;i++)vec[i].clear();
for(int i = 1;i < n;i++)
{
int u;
scanf("%d",&u);
vec[u - 1].push_back(i);
}
d[0] = 1;
q.push(0);
while(!q.empty())
{
int u = q.front();
q.pop();
for(int i = 0;i < vec[u].size();i++)
{
int v = vec[u][i];
d[v] = d[u] + 1;
q.push(v);
}
}
sort(d,d + n);
double ans = solve();
printf("%.6f\n",ans);
}
}