Problem p03166 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

/* define */
#define FOR(I, X, Y) for (long long(I) = (X); (I) < (Y); (I)++)
#define REP(I, X, Y) for (long long(I) = (Y)-1; (I) >= (X); (I)--)
#define ALL(X) (X).begin(), (X).end()
#define pb push_back
#define COUNT(V, X)                                                            \
  upper_bound((V).begin(), (V).end(), X) -                                     \
      lower_bound((V).begin(), (V).end(), X)
#define debug(x) cerr << #x << ':' << x << endl;
#define DEBUG(v)                                                               \
  cerr << #v << ':';                                                           \
  for (auto xXx : v)                                                           \
    cerr << xXx << ' ';                                                        \
  cerr << endl;
#define Yes(X)                                                                 \
  if (X) {                                                                     \
    cout << "Yes" << endl;                                                     \
  } else {                                                                     \
    cout << "No" << endl;                                                      \
  }
#define YES(X)                                                                 \
  if (X) {                                                                     \
    cout << "YES" << endl;                                                     \
  } else {                                                                     \
    cout << "NO" << endl;                                                      \
  }
#define ctoi(C) (C - '0')

/* alias */
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vii = vector<vector<int>>;
using vl = vector<long long>;
using vll = vector<vector<long long>>;
using pi = pair<int, int>;
using pl = pair<long long, long long>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const long long dx[] = {1, 0, -1, 0};
const long long dy[] = {0, 1, 0, -1};
const long long dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
const long long dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
const long long dx9[] = {1, 1, 0, -1, -1, -1, 0, 1, 0};
const int INF = 1000000007;
const long long LINF = 1000000000000000007;

/* func */
template <typename T> inline bool chmin(T &a, const T &b) {
  if (a > b)
    a = b;
  return a > b;
}
template <typename T> inline bool chmax(T &a, const T &b) {
  if (a < b)
    a = b;
  return a < b;
}

/* main */

ll N, M;
vll adj;
ll memo[100010];

ll dfs(ll n) {
  if (adj[n].size() == 0)
    return 0;
  if (memo[n])
    return memo[n];
  ll ans = 0;
  for (auto x : adj[n])
    chmax(ans, dfs(x) + 1);
  return ans;
}

signed main() {
  cin >> N >> M;
  adj.resize(N);
  FOR(i, 0, M) {
    ll x, y;
    cin >> x >> y;
    x--;
    y--;
    adj[x].pb(y);
  }
  ll ans = 0;
  FOR(i, 0, N) chmax(ans, dfs(i));
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

/* define */
#define FOR(I, X, Y) for (long long(I) = (X); (I) < (Y); (I)++)
#define REP(I, X, Y) for (long long(I) = (Y)-1; (I) >= (X); (I)--)
#define ALL(X) (X).begin(), (X).end()
#define pb push_back
#define COUNT(V, X)                                                            \
  upper_bound((V).begin(), (V).end(), X) -                                     \
      lower_bound((V).begin(), (V).end(), X)
#define debug(x) cerr << #x << ':' << x << endl;
#define DEBUG(v)                                                               \
  cerr << #v << ':';                                                           \
  for (auto xXx : v)                                                           \
    cerr << xXx << ' ';                                                        \
  cerr << endl;
#define Yes(X)                                                                 \
  if (X) {                                                                     \
    cout << "Yes" << endl;                                                     \
  } else {                                                                     \
    cout << "No" << endl;                                                      \
  }
#define YES(X)                                                                 \
  if (X) {                                                                     \
    cout << "YES" << endl;                                                     \
  } else {                                                                     \
    cout << "NO" << endl;                                                      \
  }
#define ctoi(C) (C - '0')

/* alias */
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vii = vector<vector<int>>;
using vl = vector<long long>;
using vll = vector<vector<long long>>;
using pi = pair<int, int>;
using pl = pair<long long, long long>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const long long dx[] = {1, 0, -1, 0};
const long long dy[] = {0, 1, 0, -1};
const long long dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
const long long dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
const long long dx9[] = {1, 1, 0, -1, -1, -1, 0, 1, 0};
const int INF = 1000000007;
const long long LINF = 1000000000000000007;

/* func */
template <typename T> inline bool chmin(T &a, const T &b) {
  if (a > b)
    a = b;
  return a > b;
}
template <typename T> inline bool chmax(T &a, const T &b) {
  if (a < b)
    a = b;
  return a < b;
}

/* main */

ll N, M;
vll adj;
ll memo[100010];

ll dfs(ll n) {
  if (adj[n].size() == 0)
    return 0;
  if (memo[n])
    return memo[n];
  ll ans = 0;
  for (auto x : adj[n])
    chmax(ans, dfs(x) + 1);
  return memo[n] = ans;
}

signed main() {
  cin >> N >> M;
  adj.resize(N);
  FOR(i, 0, M) {
    ll x, y;
    cin >> x >> y;
    x--;
    y--;
    adj[x].pb(y);
  }
  ll ans = 0;
  FOR(i, 0, N) chmax(ans, dfs(i));
  cout << ans << endl;
}