Problem p02902 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define endl '\n'
#define ALL(a) (a).begin(), (a).end()
#define ALLR(a) (a).rbegin(), (a).rend()
#define spa << " " <<
#define lfs << fixed << setprecision(10) <<
#define test cout << "test" << endl;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define rep(i, n, m) for (ll i = n; i < (ll)(m); i++)
#define rrep(i, n, m) for (ll i = n - 1; i >= (ll)(m); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template <typename T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <typename T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
void pmod(ll &a, ll b) { a = (a + b) % MOD; }
void pmod(ll &a, ll b, ll c) { a = (b + c) % MOD; }
void qmod(ll &a, ll b) { a = (a * b) % MOD; }
void qmod(ll &a, ll b, ll c) { a = (b * c) % MOD; }
ll median(ll a, ll b, ll c) {
  return a + b + c - max({a, b, c}) - min({a, b, c});
}
void ans1(bool x) {
  if (x)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void ans2(bool x) {
  if (x)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void ans3(bool x) {
  if (x)
    cout << "Yay!" << endl;
  else
    cout << ":(" << endl;
}
template <typename T1, typename T2> void ans(bool x, T1 y, T2 z) {
  if (x)
    cout << y << endl;
  else
    cout << z << endl;
}
template <typename T> void debug(vector<vector<T>> v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    cout << v[i][0];
    for (ll j = 1; j < w; j++)
      cout spa v[i][j];
    cout << endl;
  }
};
void debug(vector<string> v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++)
      cout << v[i][j];
    cout << endl;
  }
};
template <typename T> void debug(vector<T> v, ll n) {
  if (n != 0)
    cout << v[0];
  for (ll i = 1; i < n; i++)
    cout spa v[i];
  cout << endl;
};
template <typename T> vector<vector<T>> vec(ll x, ll y, T w) {
  vector<vector<T>> v(x, vector<T>(y, w));
  return v;
}
ll gcd(ll x, ll y) {
  ll r;
  while (y != 0 && (r = x % y) != 0) {
    x = y;
    y = r;
  }
  return y == 0 ? x : y;
}
template <typename T> vector<ll> dx = {1, 0, -1, 0, 1, 1, -1, -1};
vector<ll> dy = {0, 1, 0, -1, 1, -1, 1, -1};
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}

ll MAX_V = 2e5 + 5;
vector<vector<ll>> G(MAX_V); // 隣接リスト(添え字は始点、値は終点)

// 引数は各頂点(0-indexed)の入次数
vector<ll> tsort(vector<ll> v) {
  ll n = v.size();
  vector<ll> ret(n);
  stack<ll> st;
  for (ll i = 0; i < n; i++)
    if (v[i] == 0)
      st.push(i);
  for (ll i = 0; i < n; i++) {
    if (st.empty())
      return vector<ll>(); // 閉路がある
    ll x = st.top();
    st.pop();
    ret[x] = i;
    for (ll j = 0; j < G[x].size(); j++) {
      v[G[x][j]]--;
      if (v[G[x][j]] == 0)
        st.push(G[x][j]);
    }
  }
  return ret;
}

int main() {
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  ll res = 0, res1 = INF, res2 = -INF, buf = 0;
  bool judge = true;
  ll n, m;
  cin >> n >> m;
  vector<ll> num(n + 1);
  rep(i, 0, n) {
    ll u, v;
    cin >> u >> v;
    G[u - 1].PB(v - 1);
    num[v - 1]++;
  }
  auto v = tsort(num);
  ans(v.size() == 0, 0, -1);
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define endl '\n'
#define ALL(a) (a).begin(), (a).end()
#define ALLR(a) (a).rbegin(), (a).rend()
#define spa << " " <<
#define lfs << fixed << setprecision(10) <<
#define test cout << "test" << endl;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define rep(i, n, m) for (ll i = n; i < (ll)(m); i++)
#define rrep(i, n, m) for (ll i = n - 1; i >= (ll)(m); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template <typename T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <typename T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
void pmod(ll &a, ll b) { a = (a + b) % MOD; }
void pmod(ll &a, ll b, ll c) { a = (b + c) % MOD; }
void qmod(ll &a, ll b) { a = (a * b) % MOD; }
void qmod(ll &a, ll b, ll c) { a = (b * c) % MOD; }
ll median(ll a, ll b, ll c) {
  return a + b + c - max({a, b, c}) - min({a, b, c});
}
void ans1(bool x) {
  if (x)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void ans2(bool x) {
  if (x)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void ans3(bool x) {
  if (x)
    cout << "Yay!" << endl;
  else
    cout << ":(" << endl;
}
template <typename T1, typename T2> void ans(bool x, T1 y, T2 z) {
  if (x)
    cout << y << endl;
  else
    cout << z << endl;
}
template <typename T> void debug(vector<vector<T>> v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    cout << v[i][0];
    for (ll j = 1; j < w; j++)
      cout spa v[i][j];
    cout << endl;
  }
};
void debug(vector<string> v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++)
      cout << v[i][j];
    cout << endl;
  }
};
template <typename T> void debug(vector<T> v, ll n) {
  if (n != 0)
    cout << v[0];
  for (ll i = 1; i < n; i++)
    cout spa v[i];
  cout << endl;
};
template <typename T> vector<vector<T>> vec(ll x, ll y, T w) {
  vector<vector<T>> v(x, vector<T>(y, w));
  return v;
}
ll gcd(ll x, ll y) {
  ll r;
  while (y != 0 && (r = x % y) != 0) {
    x = y;
    y = r;
  }
  return y == 0 ? x : y;
}
template <typename T> vector<ll> dx = {1, 0, -1, 0, 1, 1, -1, -1};
vector<ll> dy = {0, 1, 0, -1, 1, -1, 1, -1};
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}

ll MAX_V = 2e5 + 5;
vector<vector<ll>> G(MAX_V); // 隣接リスト(添え字は始点、値は終点)

// 引数は各頂点(0-indexed)の入次数
vector<ll> tsort(vector<ll> v) {
  ll n = v.size();
  vector<ll> ret(n);
  stack<ll> st;
  for (ll i = 0; i < n; i++)
    if (v[i] == 0)
      st.push(i);
  for (ll i = 0; i < n; i++) {
    if (st.empty())
      return vector<ll>(); // 閉路がある
    ll x = st.top();
    st.pop();
    ret[x] = i;
    for (ll j = 0; j < G[x].size(); j++) {
      v[G[x][j]]--;
      if (v[G[x][j]] == 0)
        st.push(G[x][j]);
    }
  }
  return ret;
}

int main() {
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  ll res = 0, res1 = INF, res2 = -INF, buf = 0;
  bool judge = true;
  ll n, m;
  cin >> n >> m;
  vector<ll> num(n);
  rep(i, 0, m) {
    ll u, v;
    cin >> u >> v;
    G[u - 1].PB(v - 1);
    num[v - 1]++;
  }
  auto v = tsort(num);
  ans(v.size() == 0, 0, -1);
  return 0;
}