Problem p03166 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define REP(i, k, n) for (long long i = k; i < (long long)(n); i++)
#define all(a) a.begin(), a.end()
#define pb push_back
#define eb emplace_back
#define lb(v, k) (lower_bound(all(v), k) - v.begin())
#define ub(v, k) (upper_bound(all(v), k) - v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T, vector<T>, greater<T>>
#define dame(a)                                                                \
  {                                                                            \
    out(a);                                                                    \
    return 0;                                                                  \
  }
#define decimal cout << fixed << setprecision(15);
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> PP;
typedef tuple<ll, ll, ll, ll> PPP;
typedef multiset<ll> S;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vp = vector<P>;
using vvp = vector<vp>;
using vb = vector<bool>;
using vvb = vector<vb>;
const ll inf = 1001001001001001001;
const int INF = 1001001001;
const int mod = 1000000007;
const double eps = 1e-10;
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void out(T a) { cout << a << '\n'; }
template <class T> void outp(T a) {
  cout << '(' << a.fi << ',' << a.se << ')' << '\n';
}
template <class T> void outvp(T v) {
  rep(i, v.size()) cout << '(' << v[i].fi << ',' << v[i].se << ')';
  cout << '\n';
}
template <class T> void outvvp(T v) { rep(i, v.size()) outvp(v[i]); }
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\n';
}
template <class T> void outvv(T v) { rep(i, v.size()) outv(v[i]); }
template <class T> bool isin(T x, T l, T r) { return (l) <= (x) && (x) <= (r); }
template <class T> void yesno(T b) {
  if (b)
    out("yes");
  else
    out("no");
}
template <class T> void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
template <class T> void YESNO(T b) {
  if (b)
    out("YES");
  else
    out("NO");
}
template <class T> void noyes(T b) {
  if (b)
    out("no");
  else
    out("yes");
}
template <class T> void NoYes(T b) {
  if (b)
    out("No");
  else
    out("Yes");
}
template <class T> void NOYES(T b) {
  if (b)
    out("NO");
  else
    out("YES");
}
void outs(ll a, ll b) {
  if (a >= inf - 100)
    out(b);
  else
    out(a);
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll modpow(ll a, ll b) {
  a %= mod;
  if (b == 0)
    return 1;
  if (b & 1)
    return a * modpow(a, b - 1) % mod;
  ll k = modpow(a, b / 2);
  return k * k % mod;
}
vvi g;
vi dp;
ll dfs(int i) {
  if (dp[i] != -1)
    return dp[i];
  ll res = 0;
  for (ll x : g[i])
    chmax(res, dfs(x) + 1);
  return res;
}
int main() {
  ll n, m;
  cin >> n >> m;
  g = vvi(n);
  rep(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].pb(b);
  }
  dp = vi(n, -1);
  ll ans = 0;
  rep(i, n) chmax(ans, dfs(i));
  out(ans);
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define REP(i, k, n) for (long long i = k; i < (long long)(n); i++)
#define all(a) a.begin(), a.end()
#define pb push_back
#define eb emplace_back
#define lb(v, k) (lower_bound(all(v), k) - v.begin())
#define ub(v, k) (upper_bound(all(v), k) - v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T, vector<T>, greater<T>>
#define dame(a)                                                                \
  {                                                                            \
    out(a);                                                                    \
    return 0;                                                                  \
  }
#define decimal cout << fixed << setprecision(15);
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> PP;
typedef tuple<ll, ll, ll, ll> PPP;
typedef multiset<ll> S;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vp = vector<P>;
using vvp = vector<vp>;
using vb = vector<bool>;
using vvb = vector<vb>;
const ll inf = 1001001001001001001;
const int INF = 1001001001;
const int mod = 1000000007;
const double eps = 1e-10;
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void out(T a) { cout << a << '\n'; }
template <class T> void outp(T a) {
  cout << '(' << a.fi << ',' << a.se << ')' << '\n';
}
template <class T> void outvp(T v) {
  rep(i, v.size()) cout << '(' << v[i].fi << ',' << v[i].se << ')';
  cout << '\n';
}
template <class T> void outvvp(T v) { rep(i, v.size()) outvp(v[i]); }
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\n';
}
template <class T> void outvv(T v) { rep(i, v.size()) outv(v[i]); }
template <class T> bool isin(T x, T l, T r) { return (l) <= (x) && (x) <= (r); }
template <class T> void yesno(T b) {
  if (b)
    out("yes");
  else
    out("no");
}
template <class T> void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
template <class T> void YESNO(T b) {
  if (b)
    out("YES");
  else
    out("NO");
}
template <class T> void noyes(T b) {
  if (b)
    out("no");
  else
    out("yes");
}
template <class T> void NoYes(T b) {
  if (b)
    out("No");
  else
    out("Yes");
}
template <class T> void NOYES(T b) {
  if (b)
    out("NO");
  else
    out("YES");
}
void outs(ll a, ll b) {
  if (a >= inf - 100)
    out(b);
  else
    out(a);
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll modpow(ll a, ll b) {
  a %= mod;
  if (b == 0)
    return 1;
  if (b & 1)
    return a * modpow(a, b - 1) % mod;
  ll k = modpow(a, b / 2);
  return k * k % mod;
}
vvi g;
vi dp;
ll dfs(int i) {
  if (dp[i] != -1)
    return dp[i];
  ll res = 0;
  for (ll x : g[i])
    chmax(res, dfs(x) + 1);
  dp[i] = res;
  return res;
}
int main() {
  ll n, m;
  cin >> n >> m;
  g = vvi(n);
  rep(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].pb(b);
  }
  dp = vi(n, -1);
  ll ans = 0;
  rep(i, n) chmax(ans, dfs(i));
  out(ans);
}