Problem p03722 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

struct edge {
  ll from, to, cost;
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  vector<edge> g(n);
  rep(i, m) {
    ll a, b, c;
    cin >> a >> b >> c;
    a--, b--;
    g[i] = {a, b, -c};
  }
  vector<ll> dist(n, inf);
  dist[0] = 0;
  rep(_, 2 * n) {
    for (auto e : g) {
      if (dist[e.from] != inf) {
        chmin(dist[e.to], dist[e.from] + e.cost);
      }
    }
  }
  vector<bool> flag(n, false);
  rep(_, 2 * n) {
    for (auto e : g) {
      if (dist[e.from] != inf) {
        if (chmin(dist[e.to], dist[e.from] + e.cost)) {
          flag[e.from] = flag[e.to] = true;
        }
      }
      if (flag[e.from])
        flag[e.to] = true;
    }
  }
  if (!flag[n - 1])
    cout << -dist[n - 1] << endl;
  else
    cout << "inf" << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

struct edge {
  ll from, to, cost;
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  vector<edge> g(m);
  rep(i, m) {
    ll a, b, c;
    cin >> a >> b >> c;
    a--, b--;
    g[i] = {a, b, -c};
  }
  vector<ll> dist(n, inf);
  dist[0] = 0;
  rep(_, 2 * n) {
    for (auto e : g) {
      if (dist[e.from] != inf) {
        chmin(dist[e.to], dist[e.from] + e.cost);
      }
    }
  }
  vector<bool> flag(n, false);
  rep(_, 2 * n) {
    for (auto e : g) {
      if (dist[e.from] != inf) {
        if (chmin(dist[e.to], dist[e.from] + e.cost)) {
          flag[e.from] = flag[e.to] = true;
        }
      }
      if (flag[e.from])
        flag[e.to] = true;
    }
  }
  if (!flag[n - 1])
    cout << -dist[n - 1] << endl;
  else
    cout << "inf" << endl;
}