Problem p03754 (C++)

Buggy submission (status: Runtime Error):
#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
using ll = long long;
#define int ll
using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll LLINF = (1LL << 60);
const int INF = (1LL << 30);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  REP(i, a.size()) {
    out << a[i];
    if (i != a.size() - 1)
      out << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

VI g[100010];
signed main(void) {
  int n;
  cin >> n;
  REP(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    g[a].PB(b);
    g[b].PB(a);
  }

  vector<double> prob(n);
  function<double(int, int)> dfs1 = [&](int v, int p) -> double {
    if (p != -1 && g[v].size() == 1)
      return 0;
    int cnt = 0;
    double ret = 0;
    for (int &i : g[v]) {
      if (i == p)
        continue;
      cnt++;
      ret += dfs1(i, v) + 1;
    }
    ret /= cnt;
    return prob[v] = ret;
  };

  dfs1(0, -1);
  // cout << prob << endl;

  vector<double> ans(n);
  function<void(int, double, int)> dfs2 = [&](int v, double d_par, int p) {
    // cout << v << " " << d_par << " " << p << endl;
    // vector<PII> d_child;
    // d_child.emplace_back(0, -1);
    double ret = 0;
    for (int &e : g[v]) {
      if (e == p) {
        ret += d_par + 1;
      } else {
        ret += prob[e] + 1;
      }
    }
    // sort(d_child.rbegin(), d_child.rend());
    ans[v] = ret / g[v].size();
    for (int &e : g[v]) {
      if (e == p)
        continue;
      double nxt =
          g[v].size() == 1 ? 0 : (ret - prob[e] - 1) / (g[v].size() - 1);
      dfs2(e, nxt, v);
    }
  };

  dfs2(0, 0, -1);
  REP(i, n) { cout << fixed << setprecision(9) << ans[i] << endl; }

  return 0;
}


Fixed submission:
#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
using ll = long long;
#define int ll
using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll LLINF = (1LL << 60);
const int INF = (1LL << 30);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  REP(i, a.size()) {
    out << a[i];
    if (i != a.size() - 1)
      out << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

VI g[150010];
signed main(void) {
  int n;
  cin >> n;
  REP(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    g[a].PB(b);
    g[b].PB(a);
  }

  vector<double> prob(n);
  function<double(int, int)> dfs1 = [&](int v, int p) -> double {
    if (p != -1 && g[v].size() == 1)
      return 0;
    int cnt = 0;
    double ret = 0;
    for (int &i : g[v]) {
      if (i == p)
        continue;
      cnt++;
      ret += dfs1(i, v) + 1;
    }
    ret /= cnt;
    return prob[v] = ret;
  };

  dfs1(0, -1);
  // cout << prob << endl;

  vector<double> ans(n);
  function<void(int, double, int)> dfs2 = [&](int v, double d_par, int p) {
    // cout << v << " " << d_par << " " << p << endl;
    // vector<PII> d_child;
    // d_child.emplace_back(0, -1);
    double ret = 0;
    for (int &e : g[v]) {
      if (e == p) {
        ret += d_par + 1;
      } else {
        ret += prob[e] + 1;
      }
    }
    // sort(d_child.rbegin(), d_child.rend());
    ans[v] = ret / g[v].size();
    for (int &e : g[v]) {
      if (e == p)
        continue;
      double nxt =
          g[v].size() == 1 ? 0 : (ret - prob[e] - 1) / (g[v].size() - 1);
      dfs2(e, nxt, v);
    }
  };

  dfs2(0, 0, -1);
  REP(i, n) { cout << fixed << setprecision(9) << ans[i] << endl; }

  return 0;
}