Problem p03166 (C++)

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

using namespace std;
using ll = long long;
// #define int ll
using PII = pair<ll, ll>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }

template <typename T> 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 <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec) {
    is >> x;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  for (T i : a) {
    out << i << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0}; // DRUL
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const int MOD = 1000000007;

signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  vector<vector<ll>> g(n);
  REP(i, m) {
    ll x, y;
    cin >> x >> y;
    x--, y--;
    g[x].push_back(y);
  }

  vector<ll> dp(n, -1);
  function<ll(ll)> dfs = [&](ll v) {
    // cout << v << endl;
    if (dp[v] != -1)
      return dp[v];
    ll ret = 0;
    for (auto to : g[v]) {
      chmax(ret, dfs(to) + 1);
    }
    return ret;
  };

  ll ans = 0;
  REP(i, n) chmax(ans, dfs(i));
  cout << ans << endl;

  return 0;
}


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

using namespace std;
using ll = long long;
// #define int ll
using PII = pair<ll, ll>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }

template <typename T> 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 <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec) {
    is >> x;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  for (T i : a) {
    out << i << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0}; // DRUL
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const int MOD = 1000000007;

signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  vector<vector<ll>> g(n);
  REP(i, m) {
    ll x, y;
    cin >> x >> y;
    x--, y--;
    g[x].push_back(y);
  }

  vector<ll> dp(n, -1);
  function<ll(ll)> dfs = [&](ll v) {
    // cout << v << endl;
    if (dp[v] != -1)
      return dp[v];
    ll ret = 0;
    for (auto to : g[v]) {
      chmax(ret, dfs(to) + 1);
    }
    return dp[v] = ret;
  };

  ll ans = 0;
  REP(i, n) chmax(ans, dfs(i));
  cout << ans << endl;

  return 0;
}