Problem p01907 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
/*{{{*/ // template
#define REP(i, n) for (int i = 0; i < n; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define INF 1 << 29
#define LINF LLONG_MAX / 3
#define MP make_pair
#define PB push_back
#define pb push_back
#define EB emplace_back
#define ALL(v) (v).begin(), (v).end()
#define all(v) ALL(v)
#define sz(x) (int)(x).size()
#define debug(x) cerr << #x << ":" << x << endl
#define debug2(x, y) cerr << #x << "," << #y ":" << x << "," << y << endl
// struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream &operator<<(ostream &os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-14;
constexpr ll mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
/*}}}*/

double p;
constexpr int MN = 100001;
int N;
vector<pii> g[MN];

double poi;

double dfs1(int cur, int pre) {
  double ret = 0;
  for (int i = 0; i < g[cur].size(); i++) {
    if (g[cur][i].first == pre)
      continue;
    ret += p * (double)(g[cur][i].second + dfs1(g[cur][i].first, cur));
  }
  return ret;
}

double dfs2(int cur, int pre) {
  double ret = poi;
  for (int i = 0; i < g[cur].size(); i++) {
    if (g[cur][i].first == pre)
      continue;
    ret += p * (g[cur][i].second + dfs2(g[cur][i].first, cur));
  }
  return ret;
}

int main() {
  cin >> p;
  cin >> N;
  rep(i, N - 1) {
    int x, y, c;
    cin >> x >> y >> c;
    x--;
    y--;
    g[x].pb(MP(y, c));
    g[y].pb(MP(x, c));
  }
  poi = dfs1(0, -1);
  cout << Double(dfs2(0, -1)) << endl;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
/*{{{*/ // template
#define REP(i, n) for (int i = 0; i < n; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define INF 1 << 29
#define LINF LLONG_MAX / 3
#define MP make_pair
#define PB push_back
#define pb push_back
#define EB emplace_back
#define ALL(v) (v).begin(), (v).end()
#define all(v) ALL(v)
#define sz(x) (int)(x).size()
#define debug(x) cerr << #x << ":" << x << endl
#define debug2(x, y) cerr << #x << "," << #y ":" << x << "," << y << endl
// struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream &operator<<(ostream &os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-14;
constexpr ll mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
/*}}}*/

double p;
constexpr int MN = 100100;
int N;
vector<pii> g[MN];

double poi;

double dfs1(int cur, int paren) {
  double ret = 0;
  for (pii v : g[cur]) {
    int nx = v.first;
    int cost = v.second;
    if (nx == paren)
      continue;
    ret += p * (cost + dfs1(nx, cur));
  }
  return ret;
}

double dfs2(int cur, int paren) {
  double ret = poi;
  for (pii v : g[cur]) {
    int nx = v.first;
    int cost = v.second;
    if (nx == paren)
      continue;
    ret += p * (cost + dfs2(cost, cur));
  }
  return ret;
}

int main() {
  cin >> p >> N;
  rep(i, N - 1) {
    int cost;
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    cin >> cost;
    g[x].pb(pii(y, cost));
    g[y].pb(pii(x, cost));
  }

  poi = dfs1(0, -1);

  cout << Double(dfs2(0, -1)) << endl;
}