Problem p03166 (C++)

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

#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);

#define ll long long
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define mp make_pair
#define pu push
#define pp pop_back
#define in insert
#define ld long double
#define forn(low, high, i) for (i = low; i < high; i++)
#define forrev(high, low, i) for (i = high; i >= low; i--)
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()
#define line cout << __LINE__;
#define prv(a)                                                                 \
  for (auto x : a)                                                             \
    cout << x << ' ';                                                          \
  cout << '\n';
#define prvp(a)                                                                \
  for (auto x : a)                                                             \
    cout << "{" << x.ff << ',' << x.ss << "}";                                 \
  cout << '\n';
#define decimal_digits cout << fixed << setprecision(15);
string to_string(string s) { return '"' + s + '"'; }
string to_string(char s) { return string(1, s); }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A> string to_string(A);
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A> string to_string(A v) {
  bool f = 1;
  string r = "{";
  for (const auto &x : v) {
    if (!f)
      r += ", ";
    f = 0;
    r += to_string(x);
  }
  return r + "}";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define pr(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

typedef unordered_map<int, int> umi;
typedef unordered_map<ll, ll> uml;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int inf = 1e9;
const ll INF = 1e18;
const int mod = 1e9 + 7;
const int bit32 = log2(inf) + 3;
const int bit64 = log2(INF) + 3;

inline int add(int a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
inline int sub(int a, int b) {
  a -= b;
  if (a < 0)
    a += mod;
  return a;
}
inline int mul(int a, int b) { return (int)((long long)a * b % mod); }
inline int modexpo(int a, int b) {
  int res = 1;
  while (b > 0) {
    if (b & 1)
      res = mul(res, a);
    a = mul(a, a);
    b /= 2;
  }
  return res;
}
inline int divide(int a, int b) { return mul(a, modexpo(b, mod - 2)); }

clock_t time_p = clock();

void ktj() {
  time_p = clock() - time_p;
  cerr << "Time elapsed : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}

void pre() { // Reset each global variable (esp. for graphs)
}

const int N = 1e5 + 5;
vector<int> g[N];
int dp[N];
bool vis[N];

void dfs(int v) {
  dp[v] = 1;
  for (auto u : g[v]) {
    if (vis[u] == 0) {
      dfs(u);
    }
    dp[v] = max(dp[v], 1 + dp[u]);
  }
}

void solve() {

  int n, m;
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
  }

  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dfs(i);
    }
  }

  int ans = 0;

  for (int i = 1; i <= n; i++) {
    ans = max(ans, dp[i]);
  }

  cout << ans - 1 << '\n';
}

int main() {
  fastio

  pre();

  int t = 1;
  // cin >> t;
  while (t--) {
    solve();
  }

  ktj();
}


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

#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);

#define ll long long
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define mp make_pair
#define pu push
#define pp pop_back
#define in insert
#define ld long double
#define forn(low, high, i) for (i = low; i < high; i++)
#define forrev(high, low, i) for (i = high; i >= low; i--)
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()
#define line cout << __LINE__;
#define prv(a)                                                                 \
  for (auto x : a)                                                             \
    cout << x << ' ';                                                          \
  cout << '\n';
#define prvp(a)                                                                \
  for (auto x : a)                                                             \
    cout << "{" << x.ff << ',' << x.ss << "}";                                 \
  cout << '\n';
#define decimal_digits cout << fixed << setprecision(15);
string to_string(string s) { return '"' + s + '"'; }
string to_string(char s) { return string(1, s); }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A> string to_string(A);
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A> string to_string(A v) {
  bool f = 1;
  string r = "{";
  for (const auto &x : v) {
    if (!f)
      r += ", ";
    f = 0;
    r += to_string(x);
  }
  return r + "}";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define pr(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

typedef unordered_map<int, int> umi;
typedef unordered_map<ll, ll> uml;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int inf = 1e9;
const ll INF = 1e18;
const int mod = 1e9 + 7;
const int bit32 = log2(inf) + 3;
const int bit64 = log2(INF) + 3;

inline int add(int a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
inline int sub(int a, int b) {
  a -= b;
  if (a < 0)
    a += mod;
  return a;
}
inline int mul(int a, int b) { return (int)((long long)a * b % mod); }
inline int modexpo(int a, int b) {
  int res = 1;
  while (b > 0) {
    if (b & 1)
      res = mul(res, a);
    a = mul(a, a);
    b /= 2;
  }
  return res;
}
inline int divide(int a, int b) { return mul(a, modexpo(b, mod - 2)); }

clock_t time_p = clock();

void ktj() {
  time_p = clock() - time_p;
  cerr << "Time elapsed : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}

void pre() { // Reset each global variable (esp. for graphs)
}

const int N = 1e5 + 5;
vector<int> g[N];
int dp[N];
bool vis[N];

void dfs(int v) {
  dp[v] = 1;
  vis[v] = 1;
  for (auto u : g[v]) {
    if (vis[u] == 0) {
      dfs(u);
    }
    dp[v] = max(dp[v], 1 + dp[u]);
  }
}

void solve() {

  int n, m;
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
  }

  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dfs(i);
    }
  }

  int ans = 0;

  for (int i = 1; i <= n; i++) {
    ans = max(ans, dp[i]);
  }

  cout << ans - 1 << '\n';
}

int main() {
  fastio

  pre();

  int t = 1;
  // cin >> t;
  while (t--) {
    solve();
  }

  ktj();
}