Problem p03166 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;

#define ll long long int
#define fast ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
#define MOD 1000000007
#define pb push_back
#define S second
#define F first
#define P pair<ll, ll>
#define PI 3.1415926535897932384626433832795028
// #define ordered_set tree<ll, null_type,less_equal<>,
// rb_tree_tag,tree_order_statistics_node_update>

int mul(int a, int b) { return (long long)a * b % MOD; }

int power(int a, int b) {
  int ans = 1;
  while (b > 0) {
    if (b & 1)
      ans = mul(ans, a);
    a = mul(a, a);
    b >>= 1;
  }
  return ans;
}
// bitset<400000010> b;

// ll n=100000;
ll p[1000100] = {0};
vector<ll> v[1000000];
void sieve(ll n) {
  for (ll i = 2; i * i <= n; i++) {
    if (!p[i])
      // primes.push_back(i);
      for (ll j = i; j <= n; j += i) {
        if (i != j)
          ;
        p[j] = 1;
      }
  }
}

// ll nextPowerOf2(ll n)
// {
//     ll p = 1;
//     if (n && !(n & (n - 1)))
//         return n;

//     while (p < n)
//         p <<= 1;

//     return p;
// }
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll factorial(ll n) {
  if (n == 0)
    return 1;
  return n * factorial(n - 1);
}
struct segmenttree {
  vector<int> st;
  int n;
  void init(int _n) {
    n = _n;
    st.clear();
    st.resize(4 * _n, 0);
  }

  void update(int l, int r, int indup, int val, int node) {
    if (l == r) {
      st[node] += val;
      return;
    } else {
      int mid = (l + r) / 2;
      if (indup >= l && indup <= mid) {
        update(l, mid, indup, val, node * 2 + 1);
      } else {
        update(mid + 1, r, indup, val, node * 2 + 2);
      }
      st[node] = st[2 * node + 1] + st[2 * node + 2];
    }
  }

  int query(int si, int se, int l, int r, int node) {
    if (se < l || si > r || l > r) {
      return 0;
    }
    if (si >= l && se <= r) {
      return st[node];
    }
    int mid = (si + se) / 2;
    int q1 = query(si, mid, l, r, node * 2 + 1);
    int q2 = query(mid + 1, se, l, r, node * 2 + 2);
    return q1 + q2;
  }
  int query(int l, int r) { return query(0, n - 1, l, r, 0); }
  void update(int index, int val) { update(0, n - 1, index, val, 0); }
} tree;

bool chkcow(ll n, vector<ll> &a, ll k, ll mis) {
  ll sum = 0;
  for (ll i = 0; i < n; i++) {
    if (a[i] > mis)
      sum += a[i] - mis;
    if (sum >= k)
      return true;
  }

  return false;
}
struct cmp {
  bool operator()(const pair<ll, ll> &a, const pair<ll, ll> &b) const {
    ll lena = a.S - a.F;
    ll lenb = b.S - b.F;
    if (lena == lenb)
      return a.F < b.F;
    return lena > lenb;
  }
};

vector<ll> graph[100000];
ll dp[100001];

ll solv(ll node) {
  if (dp[node] != -1)
    return dp[node];
  bool leaf = 1;
  ll best = 0;
  for (auto child : graph[node]) {
    leaf = 0;
    best = max(best, solv(child));
  }
  return leaf ? 0 : 1 + best;
}

int32_t main() {
  fast

      // #ifndef ONLINE_JUDGE
      // 	freopen("input.txt", "r", stdin);
      // 	freopen("output.txt", "w", stdout);
      // #endif
      // ll t;
      // cin >> t;
      // while (t--) {
      ll n,
      m;
  cin >> n >> m;
  memset(dp, -1, sizeof dp);
  for (ll i = 0; i < m; i++) {
    ll u, v;
    cin >> u >> v;
    graph[u].pb(v);
  }
  ll ans = 0;
  for (ll i = 1; i <= n; i++) {
    ans = max(ans, solv(i));
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;

#define ll long long int
#define fast ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
#define MOD 1000000007
#define pb push_back
#define S second
#define F first
#define P pair<ll, ll>
#define PI 3.1415926535897932384626433832795028
// #define ordered_set tree<ll, null_type,less_equal<>,
// rb_tree_tag,tree_order_statistics_node_update>

int mul(int a, int b) { return (long long)a * b % MOD; }

int power(int a, int b) {
  int ans = 1;
  while (b > 0) {
    if (b & 1)
      ans = mul(ans, a);
    a = mul(a, a);
    b >>= 1;
  }
  return ans;
}
// bitset<400000010> b;

// ll n=100000;
ll p[1000100] = {0};
vector<ll> v[1000000];
void sieve(ll n) {
  for (ll i = 2; i * i <= n; i++) {
    if (!p[i])
      // primes.push_back(i);
      for (ll j = i; j <= n; j += i) {
        if (i != j)
          ;
        p[j] = 1;
      }
  }
}

// ll nextPowerOf2(ll n)
// {
//     ll p = 1;
//     if (n && !(n & (n - 1)))
//         return n;

//     while (p < n)
//         p <<= 1;

//     return p;
// }
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll factorial(ll n) {
  if (n == 0)
    return 1;
  return n * factorial(n - 1);
}
struct segmenttree {
  vector<int> st;
  int n;
  void init(int _n) {
    n = _n;
    st.clear();
    st.resize(4 * _n, 0);
  }

  void update(int l, int r, int indup, int val, int node) {
    if (l == r) {
      st[node] += val;
      return;
    } else {
      int mid = (l + r) / 2;
      if (indup >= l && indup <= mid) {
        update(l, mid, indup, val, node * 2 + 1);
      } else {
        update(mid + 1, r, indup, val, node * 2 + 2);
      }
      st[node] = st[2 * node + 1] + st[2 * node + 2];
    }
  }

  int query(int si, int se, int l, int r, int node) {
    if (se < l || si > r || l > r) {
      return 0;
    }
    if (si >= l && se <= r) {
      return st[node];
    }
    int mid = (si + se) / 2;
    int q1 = query(si, mid, l, r, node * 2 + 1);
    int q2 = query(mid + 1, se, l, r, node * 2 + 2);
    return q1 + q2;
  }
  int query(int l, int r) { return query(0, n - 1, l, r, 0); }
  void update(int index, int val) { update(0, n - 1, index, val, 0); }
} tree;

bool chkcow(ll n, vector<ll> &a, ll k, ll mis) {
  ll sum = 0;
  for (ll i = 0; i < n; i++) {
    if (a[i] > mis)
      sum += a[i] - mis;
    if (sum >= k)
      return true;
  }

  return false;
}
struct cmp {
  bool operator()(const pair<ll, ll> &a, const pair<ll, ll> &b) const {
    ll lena = a.S - a.F;
    ll lenb = b.S - b.F;
    if (lena == lenb)
      return a.F < b.F;
    return lena > lenb;
  }
};

vector<ll> graph[100000];
ll dp[100001];

ll solv(ll node) {
  if (dp[node] != -1)
    return dp[node];
  bool leaf = 1;
  ll best = 0;
  for (auto child : graph[node]) {
    leaf = 0;
    best = max(best, solv(child));
  }
  return dp[node] = leaf ? 0 : 1 + best;
}

int32_t main() {
  fast

      // #ifndef ONLINE_JUDGE
      // 	freopen("input.txt", "r", stdin);
      // 	freopen("output.txt", "w", stdout);
      // #endif
      // ll t;
      // cin >> t;
      // while (t--) {
      ll n,
      m;
  cin >> n >> m;
  memset(dp, -1, sizeof dp);
  for (ll i = 0; i < m; i++) {
    ll u, v;
    cin >> u >> v;
    graph[u].pb(v);
  }
  ll ans = 0;
  for (ll i = 1; i <= n; i++) {
    ans = max(ans, solv(i));
  }
  cout << ans << endl;
  return 0;
}