Problem p03166 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repo(i, n) for (int i = 1; i < (int)(n); i++)
#define pb push_back
#define mp make_pair
#define np next_permutation
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define mod 1000000007
#define pi acos(-1.0)
const ll INF = 1LL << 61;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
ll jou(ll N, ll P) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = jou(N, P / 2);
    return t * t % mod;
  }
  return (N * jou(N, P - 1)) % mod;
} // jou(n,mod-2)で逆元
// intの最大値2147483647 ≒ 2×10^9
// long longの最大値9223372036854775807 ≒ 9×10^18
//'大文字'+=32;    で小文字に
//   cout << fixed << setprecision (20);   小数点以下2０桁まで
// 実行時間制約2秒では２×10^8回くらいまで計算できる

int len(int x, vector<vector<int>> &p, vector<int> &memo) {
  if (memo[x] != -1)
    return memo[x];
  if (p[x].empty())
    return 0;
  int rtn = 0;
  for (auto z : p[x]) {
    chmax(rtn, len(z, p, memo) + 1);
  }
  return rtn;
}

int main() {
  int n, m;
  cin >> n >> m;

  vector<vector<int>> p(n);
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    p[x].pb(y);
  }

  vector<int> memo(n, -1);

  int ans = 0;
  rep(i, n) { chmax(ans, len(i, p, memo)); }

  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repo(i, n) for (int i = 1; i < (int)(n); i++)
#define pb push_back
#define mp make_pair
#define np next_permutation
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define mod 1000000007
#define pi acos(-1.0)
const ll INF = 1LL << 61;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
ll jou(ll N, ll P) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = jou(N, P / 2);
    return t * t % mod;
  }
  return (N * jou(N, P - 1)) % mod;
} // jou(n,mod-2)で逆元
// intの最大値2147483647 ≒ 2×10^9
// long longの最大値9223372036854775807 ≒ 9×10^18
//'大文字'+=32;    で小文字に
//   cout << fixed << setprecision (20);   小数点以下2０桁まで
// 実行時間制約2秒では２×10^8回くらいまで計算できる

int len(int x, vector<vector<int>> &p, vector<int> &memo) {
  if (memo[x] != -1)
    return memo[x];
  if (p[x].empty())
    return 0;
  int rtn = 0;
  for (auto z : p[x]) {
    chmax(rtn, len(z, p, memo) + 1);
  }
  memo[x] = rtn;
  return rtn;
}

int main() {
  int n, m;
  cin >> n >> m;

  vector<vector<int>> p(n);
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    p[x].pb(y);
  }

  vector<int> memo(n, -1);

  int ans = 0;
  rep(i, n) { chmax(ans, len(i, p, memo)); }

  cout << ans << endl;
}