Problem p03055 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep1(i, n) for (int i = 1; i < (int)(n); ++i)
#define repeq(i, n) for (int i = 0; i <= (int)(n); ++i)
#define rep1eq(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rrep1(i, n) for (int i = (int)(n)-1; i > 0; --i)
#define rrepeq(i, n) for (int i = (int)(n); i >= 0; --i)
#define rrep1eq(i, n) for (int i = (int)(n); i > 0; --i)
#define REP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a); i >= (int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template <typename T> using Graph = vector<vector<T>>;
template <typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9 + 7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template <typename T> struct is_plural : false_type {};
template <typename T1, typename T2>
struct is_plural<pair<T1, T2>> : true_type {};
template <typename T> struct is_plural<vector<T>> : true_type {};
template <typename T> struct is_plural<complex<T>> : true_type {};

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  return is >> p.first >> p.second;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto itr = vec.begin(); itr != vec.end(); ++itr)
    is >> *itr;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  if (vec.empty())
    return os;
  bool pl = is_plural<T>();
  os << vec.front();
  for (auto itr = ++vec.begin(); itr != vec.end(); ++itr)
    os << interval[pl] << *itr;
  return os;
}

bool CoutYN(bool a, string y = "Yes", string n = "No") {
  cout << (a ? y : n) << "\n";
  return a;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

long long modpow(int a, long long n, int mod = MOD) {
  long long ret = 1;
  do {
    if (n & 1)
      ret = ret * a % mod;
    a = 1LL * a * a % mod;
  } while (n >>= 1);
  return ret;
}

template <typename T> T GCD(T a, T b) { return b ? GCD(b, a % b) : a; }
template <typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template <typename T1, typename T2>
bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) {
  return a.second != b.second ? a.second < b.second : a.first < b.first;
}
template <typename T1, typename T2>
bool CompareByInverse(pair<T1, T2> a, pair<T1, T2> b) {
  return a.first != b.first ? a.first < b.first : a.second > b.second;
}

/* -------- <templates end> -------- */

void solve() {
  int n;
  cin >> n;
  Graph<int> g(n);
  rep1(_, n) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }

  int s, t;
  int MAX = 0;

  auto dfs = [&](auto dfs, int v, int p, int d, int &x) -> void {
    if (chmax(MAX, d))
      x = v;
    for (auto &nv : g[v]) {
      if (nv != p) {
        dfs(dfs, nv, v, d + 1, x);
      }
    }
  };

  dfs(dfs, 0, -1, 0, s);
  MAX = 0;
  dfs(dfs, s, -1, 0, t);

  CoutYN(MAX % 3 == 1, "Second", "First");
}

/* -------- <programs end> -------- */

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep1(i, n) for (int i = 1; i < (int)(n); ++i)
#define repeq(i, n) for (int i = 0; i <= (int)(n); ++i)
#define rep1eq(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rrep1(i, n) for (int i = (int)(n)-1; i > 0; --i)
#define rrepeq(i, n) for (int i = (int)(n); i >= 0; --i)
#define rrep1eq(i, n) for (int i = (int)(n); i > 0; --i)
#define REP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a); i >= (int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template <typename T> using Graph = vector<vector<T>>;
template <typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9 + 7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template <typename T> struct is_plural : false_type {};
template <typename T1, typename T2>
struct is_plural<pair<T1, T2>> : true_type {};
template <typename T> struct is_plural<vector<T>> : true_type {};
template <typename T> struct is_plural<complex<T>> : true_type {};

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  return is >> p.first >> p.second;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto itr = vec.begin(); itr != vec.end(); ++itr)
    is >> *itr;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  if (vec.empty())
    return os;
  bool pl = is_plural<T>();
  os << vec.front();
  for (auto itr = ++vec.begin(); itr != vec.end(); ++itr)
    os << interval[pl] << *itr;
  return os;
}

bool CoutYN(bool a, string y = "Yes", string n = "No") {
  cout << (a ? y : n) << "\n";
  return a;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

long long modpow(int a, long long n, int mod = MOD) {
  long long ret = 1;
  do {
    if (n & 1)
      ret = ret * a % mod;
    a = 1LL * a * a % mod;
  } while (n >>= 1);
  return ret;
}

template <typename T> T GCD(T a, T b) { return b ? GCD(b, a % b) : a; }
template <typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template <typename T1, typename T2>
bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) {
  return a.second != b.second ? a.second < b.second : a.first < b.first;
}
template <typename T1, typename T2>
bool CompareByInverse(pair<T1, T2> a, pair<T1, T2> b) {
  return a.first != b.first ? a.first < b.first : a.second > b.second;
}

/* -------- <templates end> -------- */

void solve() {
  int n;
  cin >> n;

  if (n == 1) {
    cout << "First\n";
    return;
  }

  Graph<int> g(n);
  rep1(_, n) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }

  int s, t;
  int MAX = 0;

  auto dfs = [&](auto dfs, int v, int p, int d, int &x) -> void {
    if (chmax(MAX, d))
      x = v;
    for (auto &nv : g[v]) {
      if (nv != p) {
        dfs(dfs, nv, v, d + 1, x);
      }
    }
  };

  dfs(dfs, 0, -1, 0, s);
  MAX = 0;
  dfs(dfs, s, -1, 0, t);

  CoutYN(MAX % 3 == 1, "Second", "First");
}

/* -------- <programs end> -------- */

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}