Problem p02884 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define MOD (long long int)(1e9 + 7)
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  ret *= kaijo(n);
  ret %= MOD;
  ret *= gyaku_kaijo(r);
  ret %= MOD;
  ret *= gyaku_kaijo(n - r);
  ret %= MOD;
  return ret;
}

int n, m;
list<int> G[1000];
double dp[1000], p[1000], dp2[1000];
void func(int now) {
  if (now == n - 1) {
    rep(i, n) {
      dp[i] = 0.0;
      p[i] = 0.0;
    }
    p[0] = 1.0;
  }

  for (auto itr = G[now].begin(); itr != G[now].end(); itr++) {
    int to = *itr;
    dp[now] += (dp[to] + 1.0) / G[now].size();
  }
  if (now != 0)
    func(now - 1);
}

void func2(int now, double dp_val, bool init) {
  if (init) {
    rep(i, n) { dp2[i] = dp[i]; }
    dp2[now] = dp_val;
    func2(now - 1, 0, false);
    return;
  }

  if (now == -1)
    return;

  dp2[now] = 0.0;
  for (auto itr = G[now].begin(); itr != G[now].end(); itr++) {
    int to = *itr;
    dp2[now] += (dp2[to] + 1.0) / G[now].size();
  }
  func2(now - 1, 0, false);
}

int main(void) {
  cin >> n >> m;
  rep(i, m) {
    ll s, t;
    cin >> s >> t;
    s--;
    t--;
    G[s].push_back(t);
  }
  func(n - 1);
  double ans = dp[0];
  rep(i, n) {
    int best_to = -1;
    double best = -1;
    if (G[i].size() <= 1)
      continue;
    for (auto itr = G[i].begin(); itr != G[i].end(); itr++) {
      int to = *itr;
      if (dp[to] > best) {
        best_to = to;
        best = dp[to];
      }
    }

    double sum_new = 0.0;
    for (auto itr = G[i].begin(); itr != G[i].end(); itr++) {
      int to = *itr;
      if (best_to == to)
        continue;
      sum_new += (dp[to] + 1.0) / (G[i].size() - 1.0);
    }
    func2(i, sum_new, true);

    chmin(ans, dp2[0]);
  }
  cout << setprecision(10) << ans << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define MOD (long long int)(1e9 + 7)
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  ret *= kaijo(n);
  ret %= MOD;
  ret *= gyaku_kaijo(r);
  ret %= MOD;
  ret *= gyaku_kaijo(n - r);
  ret %= MOD;
  return ret;
}

int n, m;
vector<int> G[1000];
double dp[1000], p[1000], dp2[1000];
void func(int now) {
  if (now == n - 1) {
    rep(i, n) {
      dp[i] = 0.0;
      p[i] = 0.0;
    }
    p[0] = 1.0;
  }

  for (auto itr = G[now].begin(); itr != G[now].end(); itr++) {
    int to = *itr;
    dp[now] += (dp[to] + 1.0) / G[now].size();
  }
  if (now != 0)
    func(now - 1);
}

void func2(int now, double dp_val, bool init) {
  if (init) {
    rep(i, n) { dp2[i] = dp[i]; }
    dp2[now] = dp_val;
    func2(now - 1, 0, false);
    return;
  }

  if (now == -1)
    return;

  dp2[now] = 0.0;
  for (auto itr = G[now].begin(); itr != G[now].end(); itr++) {
    int to = *itr;
    dp2[now] += (dp2[to] + 1.0) / G[now].size();
  }
  func2(now - 1, 0, false);
}

int main(void) {
  cin >> n >> m;
  rep(i, m) {
    ll s, t;
    cin >> s >> t;
    s--;
    t--;
    G[s].push_back(t);
  }
  func(n - 1);
  double ans = dp[0];
  rep(i, n) {
    int best_to = -1;
    double best = -1;
    if (G[i].size() <= 1)
      continue;
    for (auto itr = G[i].begin(); itr != G[i].end(); itr++) {
      int to = *itr;
      if (dp[to] > best) {
        best_to = to;
        best = dp[to];
      }
    }

    double sum_new = 0.0;
    for (auto itr = G[i].begin(); itr != G[i].end(); itr++) {
      int to = *itr;
      if (best_to == to)
        continue;
      sum_new += (dp[to] + 1.0) / (G[i].size() - 1.0);
    }
    func2(i, sum_new, true);

    chmin(ans, dp2[0]);
  }
  cout << setprecision(10) << ans << endl;
  return 0;
}