Problem p02884 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename F> struct FixPoint : F {
  explicit FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

int main() {
  int N, M;
  cin >> N >> M;

  auto dp = make_v<double>(N, N + 1);
  fill_v(dp, infll);
  UnWeightedGraph g(N);
  auto rec = MFP([&](auto rec, int idx) -> void {
    if (dp[idx][0] < infll) {
      return;
    }
    if (idx + 1 == N) {
      dp[idx][N] = 0.0;
      return;
    }

    for (auto &to : g[idx])
      rec(to);

    int sz = (int)g[idx].size();

    {
      double ret = 0.0;
      for (auto &to : g[idx])
        ret += dp[to][N];
      dp[idx][N] = ret / sz + 1;
    }

    {
      for (int k = idx + 1; k < N; k++) {
        double ret = 0.0;
        for (auto &to : g[idx])
          ret += min(dp[to][N], dp[to][k]);
        chmin(dp[idx][k], ret / sz + 1);
      }
    }

    if (sz > 1) {
      double ret = 0.0;
      for (auto &to : g[idx])
        ret += dp[to][N];
      for (auto &to : g[idx])
        chmin(dp[idx][idx], (ret - dp[to][N]) / (sz - 1) + 1);
    }
  });
  for (int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    g[a].emplace_back(b);
  }
  rec(0);
  cout << *min_element(begin(dp[0]), end(dp[0])) << endl;
}


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

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename F> struct FixPoint : F {
  explicit FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

int main() {
  int N, M;
  cin >> N >> M;

  auto dp = make_v<double>(N, N + 1);
  fill_v(dp, infll);
  UnWeightedGraph g(N);
  auto rec = MFP([&](auto rec, int idx) -> void {
    if (dp[idx][N] < infll) {
      return;
    }
    if (idx + 1 == N) {
      dp[idx][N] = 0.0;
      return;
    }

    for (auto &to : g[idx])
      rec(to);

    int sz = (int)g[idx].size();

    {
      double ret = 0.0;
      for (auto &to : g[idx])
        ret += dp[to][N];
      dp[idx][N] = ret / sz + 1;
    }

    {
      for (int k = idx + 1; k < N; k++) {
        double ret = 0.0;
        for (auto &to : g[idx])
          ret += min(dp[to][N], dp[to][k]);
        chmin(dp[idx][k], ret / sz + 1);
      }
    }

    if (sz > 1) {
      double ret = 0.0;
      for (auto &to : g[idx])
        ret += dp[to][N];
      for (auto &to : g[idx])
        chmin(dp[idx][idx], (ret - dp[to][N]) / (sz - 1) + 1);
    }
  });
  for (int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    g[a].emplace_back(b);
  }
  rec(0);
  cout << *min_element(begin(dp[0]), end(dp[0])) << endl;
}