hdu 5425 Rikka with Tree II（暴力）

题目链接：hdu 5425 Rikka with Tree II

直接枚举就好了，当概率极小时贡献值可以忽略。

#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <queue>
#include <algorithm>

using namespace std;

const int maxn = 1e5 + 5;

int N, D[maxn];
vector<int> G[maxn];

void init () {
for (int i = 1; i <= N; i++) G[i].clear();

int t;
for (int i = 2; i <= N; i++) {
scanf("%d", &t);
G[t].push_back(i);
}

queue<int> Q;
D[1] = 0;
Q.push(1);

while (!Q.empty()) {
int u = Q.front();
Q.pop();

for (int i = 0; i < G[u].size(); i++) {
int v = G[u][i];
D[v] = D[u] + 1;
Q.push(v);
}
}
sort(D + 1, D + 1 + N);
}

int main () {
while (scanf("%d", &N) == 1) {
init ();

double ans = 0;
for (int i = 0; i < N && i < 100; i++) {
int u = N - i;
double w = 0;
for (int j = N; j > u; j--)
w += 1.0 * (D[u] + 1) * (D[j] + 1) / (D[u] + D[j] + 2);
double t = pow(2, N-u+1) - (N+1) / pow(2, u-1);
ans += w / t;
}
printf("%.6lf\n", ans);
}
return 0;
}