Problem p02371 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it, X) for (auto it = (X).begin(); it != (X).end(); it++)
#define numa(x, a) for (auto x : a)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i, n) rep2(i, 0, n)
#define rep2(i, m, n) for (int i = m; i < (n); i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL(c) (c).begin(), (c).end()
#define gcd(a, b) __gcd(a, b)
#define endl "\\n"
using namespace std;
template <int POS, class TUPLE>
void deploy(std::ostream &os, const TUPLE &tuple) {}
template <int POS, class TUPLE, class H, class... Ts>
void deploy(std::ostream &os, const TUPLE &t) {
  os << (POS == 0 ? "" : ", ") << get<POS>(t);
  deploy<POS + 1, TUPLE, Ts...>(os, t);
}
template <class T>
std::ostream &operator<<(std::ostream &os, std::vector<T> &v) {
  int remain = v.size();
  os << "{";
  for (auto e : v)
    os << e << (--remain == 0 ? "}" : ", ");
  return os;
}
template <class T> std::ostream &operator<<(std::ostream &os, std::set<T> &v) {
  int remain = v.size();
  os << "{";
  for (auto e : v)
    os << e << (--remain == 0 ? "}" : ", ");
  return os;
}
template <class T, class K>
std::ostream &operator<<(std::ostream &os, std::map<T, K> &mp) {
  int remain = mp.size();
  os << "{";
  for (auto e : mp)
    os << "(" << e.first << " -> " << e.second << ")"
       << (--remain == 0 ? "}" : ", ");
  return os;
}
#define DEBUG1(var0)                                                           \\
  { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1)                                                     \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG1(var1);                                                              \\
  }
#define DEBUG3(var0, var1, var2)                                               \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG2(var1, var2);                                                        \\
  }
#define DEBUG4(var0, var1, var2, var3)                                         \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG3(var1, var2, var3);                                                  \\
  }
typedef long long ll;

#define MAX_N 100100
int V;
vector<pair<int, ll>> edge[MAX_N];
ll ans;
bool used[MAX_N];
ll kihaba(int now) {
  if (used[now]) {
    return 0;
  }
  used[now] = true;
  vector<ll> memo;
  rep(i, sz(edge[now])) {
    if (!used[edge[now][i].fir]) {
      memo.pb(kihaba(edge[now][i].fir) + edge[now][i].sec);
    }
  }
  sort(ALL(memo));
  reverse(ALL(memo));
  if (sz(memo) >= 2) {
    ans = max(ans, memo[0] + memo[1]);
  }
  if (sz(memo) >= 1) {
    ans = max(ans, memo[0]);
    return memo[0];
  } else {
    return 0;
  }
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> V;
  rep(i, V - 1) {
    int x, y;
    ll w;
    cin >> x >> y >> w;
    edge[x].pb(mp(y, w));
    edge[y].pb(mp(x, w));
  }
  rep(i, V) { kihaba(i); }
  cout << ans << endl;
  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it, X) for (auto it = (X).begin(); it != (X).end(); it++)
#define numa(x, a) for (auto x : a)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i, n) rep2(i, 0, n)
#define rep2(i, m, n) for (int i = m; i < (n); i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL(c) (c).begin(), (c).end()
#define gcd(a, b) __gcd(a, b)
#define endl "\\n"
using namespace std;
template <int POS, class TUPLE>
void deploy(std::ostream &os, const TUPLE &tuple) {}
template <int POS, class TUPLE, class H, class... Ts>
void deploy(std::ostream &os, const TUPLE &t) {
  os << (POS == 0 ? "" : ", ") << get<POS>(t);
  deploy<POS + 1, TUPLE, Ts...>(os, t);
}
template <class T>
std::ostream &operator<<(std::ostream &os, std::vector<T> &v) {
  int remain = v.size();
  os << "{";
  for (auto e : v)
    os << e << (--remain == 0 ? "}" : ", ");
  return os;
}
template <class T> std::ostream &operator<<(std::ostream &os, std::set<T> &v) {
  int remain = v.size();
  os << "{";
  for (auto e : v)
    os << e << (--remain == 0 ? "}" : ", ");
  return os;
}
template <class T, class K>
std::ostream &operator<<(std::ostream &os, std::map<T, K> &mp) {
  int remain = mp.size();
  os << "{";
  for (auto e : mp)
    os << "(" << e.first << " -> " << e.second << ")"
       << (--remain == 0 ? "}" : ", ");
  return os;
}
#define DEBUG1(var0)                                                           \\
  { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1)                                                     \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG1(var1);                                                              \\
  }
#define DEBUG3(var0, var1, var2)                                               \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG2(var1, var2);                                                        \\
  }
#define DEBUG4(var0, var1, var2, var3)                                         \\
  {                                                                            \\
    std::cerr << (#var0) << "=" << (var0) << ", ";                             \\
    DEBUG3(var1, var2, var3);                                                  \\
  }
typedef long long ll;

#define MAX_N 100100
int V;
vector<pair<int, ll>> edge[MAX_N];
ll ans;
ll kihaba(int before, int now) {
  if (sz(edge[now]) == 1) {
    return 0;
  }
  vector<ll> memo;
  rep(i, sz(edge[now])) {
    if (edge[now][i].fir != before) {
      memo.pb(kihaba(now, edge[now][i].fir) + edge[now][i].sec);
    }
  }
  sort(ALL(memo));
  reverse(ALL(memo));
  if (sz(memo) >= 2) {
    ans = max(ans, memo[0] + memo[1]);
  }
  return memo[0];
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> V;
  rep(i, V - 1) {
    int x, y;
    ll w;
    cin >> x >> y >> w;
    edge[x].pb(mp(y, w));
    edge[y].pb(mp(x, w));
  }
  kihaba(-1, 1);
  cout << ans << endl;
  return 0;
}