Problem p03055 (C++)

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

using ll = long long;
#define rng(i, l, r) for (int i = int(l); i < int(r); i++)
#define rep(i, r) rng(i, 0, r)
#define rrng(i, l, r) for (int i = int(r) - 1; i >= int(l); i--)
#define rrep(i, r) rrng(i, 0, r)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define one(x) memset(x, -1, sizeof(x))
#define zero(x) memset(x, 0, sizeof(x))
#define F first
#define S second
#define bg begin()
#define ed end()
#define all(x) x.bg, x.ed
#define si(x) int(x.size())
#define inf INT_MAX / 2 - 100
#define infl LLONG_MAX / 3
#ifdef LOCAL
#define dmp(x) cerr << __LINE__ << ' ' << #x << ' ' << x << endl
#else
#define dmp(x) void(0)
#endif

template <class t, class u> void chmax(t &a, u b) {
  if (a < b)
    a = b;
}
template <class t, class u> void chmin(t &a, u b) {
  if (b < a)
    a = b;
}

template <class t> using vc = vector<t>;
template <class t> using vvc = vector<vector<t>>;

using pi = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vc<int>;
using vl = vc<ll>;

ll readl(void) {
  ll x;
  cin >> x;
  return x;
}
int readi(void) {
  int x;
  cin >> x;
  return x;
}

string readstr() {
  string s;
  cin >> s;
  return s;
}
vi readvi(int n, int off = 0) {
  vi v(n);
  rep(i, n) v[i] = readi(), v[i] += off;
  return v;
}

vl readvl(int n, int off = 0) {
  vl v(n);
  rep(i, n) v[i] = readl(), v[i] += off;
  return v;
}

template <class t> void print(t x, int suc = 1) {
  cout << x;
  if (suc == 1)
    cout << "\n";
  if (suc == 2)
    cout << " ";
}

template <class t> void print(const vc<t> &v, int suc = 1) {
  rep(i, si(v)) print(v[i], i == int(si(v)) - 1 ? 1 : suc);
}

template <class t> bool inc(t a, t b, t c) { return !(c < b || b < a); }

template <class t> void compress(vc<t> &v) {
  sort(all(v));
  v.erase(unique(all(v)), v.ed);
}

template <class t> int lwb(const vc<t> &v, const t &a) {
  return lower_bound(all(v), a) - v.bg;
}

template <class t> struct Compress {
  vc<t> v;
  Compress() = default;
  Compress(const vc<t> &x) { add(x); }
  Compress(const initializer_list<vc<t>> &x) {
    for (auto &p : x)
      add(p);
  }
  void add(const t &x) { v.eb(x); }
  void add(const vc<t> &x) { copy(all(x), back_inserter(v)); }
  void build() { compress(v); }
  int get(const t &x) const { return lwb(v, x); }
  vc<t> get(const vc<t> &x) const {
    vc<t> res(x);
    for (auto &p : res)
      p = get(p);
    return res;
  }
  const t &operator[](int x) const { return v[x]; }
};
void Yes(bool ex = true) {
  cout << "Yes\n";
  if (ex)
    exit(0);
}
void YES(bool ex = true) {
  cout << "YES\n";
  if (ex)
    exit(0);
}
void No(bool ex = true) {
  cout << "No\n";
  if (ex)
    exit(0);
}
void NO(bool ex = true) {
  cout << "NO\n";
  if (ex)
    exit(0);
}
void orYes(bool x, bool ex = true) {
  if (x)
    Yes(ex);
  else
    No(ex);
}
void orYES(bool x, bool ex = true) {
  if (x)
    YES(ex);
  else
    NO(ex);
}
void Possible(bool ex = true) {
  cout << "Possible\n";
  if (ex)
    exit(0);
}
void POSSIBLE(bool ex = true) {
  cout << "POSSIBLE\n";
  if (ex)
    exit(0);
}
void Impossible(bool ex = true) {
  cout << "Impossible\n";
  if (ex)
    exit(0);
}
void IMPOSSIBLE(bool ex = true) {
  cout << "IMPOSSIBLE\n";
  if (ex)
    exit(0);
}
void orPossible(bool x, bool ex = true) {
  if (x)
    Possible(ex);
  else
    Impossible(ex);
}
void orPOSSIBLE(bool x, bool ex = true) {
  if (x)
    POSSIBLE(ex);
  else
    IMPOSSIBLE(ex);
}
using uint = unsigned;
using ull = unsigned long long;

template <uint const &MOD> struct Modular {
  static constexpr uint const &mod = MOD;
  uint v;
  Modular(long long x = 0) { c(x % mod + mod); }
  Modular &c(uint x) {
    v = x < mod ? x : x - mod;
    return *this;
  }
  Modular pow(int k) const {
    Modular res(1), tmp(v);
    while (k) {
      if (k & 1)
        res *= tmp;
      tmp *= tmp;
      k >>= 1;
    }
    return res;
  }

  Modular inv() const { return pow(mod - 2); }
  Modular operator-() const { return Modular(mod - v); }

  Modular &operator+=(const Modular &x) { return c(v + x.v); }
  Modular &operator-=(const Modular &x) { return c(v + mod - x.v); }
  Modular &operator*=(const Modular &x) {
    v = ull(v) * x.v % mod;
    return *this;
  }
  Modular &operator/=(const Modular &x) { return *this *= x.inv(); }

  Modular operator+(const Modular &x) const { return Modular(*this) += x; }
  Modular operator-(const Modular &x) const { return Modular(*this) -= x; }
  Modular operator*(const Modular &x) const { return Modular(*this) *= x; }
  Modular operator/(const Modular &x) const { return Modular(*this) /= x; }

  friend Modular operator+(long long x, const Modular &y) {
    return Modular(x) + y;
  }
  friend Modular operator-(long long x, const Modular &y) {
    return Modular(x) - y;
  }
  friend Modular operator*(long long x, const Modular &y) {
    return Modular(x) * y;
  }
  friend Modular operator/(long long x, const Modular &y) {
    return Modular(x) / y;
  }

  friend ostream &operator<<(ostream &os, const Modular &x) {
    return os << x.v;
  }
  friend istream &operator>>(istream &is, Modular &x) {
    long long p;
    is >> p;
    x = Modular(p);
    return is;
  }
  bool operator==(const Modular &x) const { return v == x.v; }
  bool operator!=(const Modular &x) const { return v != x.v; }
  bool operator<(const Modular &x) const { return v < x.v; }
  explicit operator bool() const { return v; }
};

vi g[100005];
int d[100005];
void dfs(int v, int p, int x) {
  d[v] = x;
  for (auto &i : g[v]) {
    if (i != p) {
      dfs(i, v, x + 1);
    }
  }
}
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(0);
  int n;
  cin >> n;
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    g[--a].pb(--b);
    g[b].pb(a);
  }
  dfs(0, -1, 0);
  pi ma = mp(0, 0);
  rep(i, n) { chmax(ma, mp(d[i], i)); }
  dfs(ma.S, -1, 0);
  ma = mp(0, ma.S);
  rep(i, n) { chmax(ma, mp(d[i], i)); }
  if ((ma.F + 1) % 3 == 2)
    print("Second");
  else
    print("First");
}


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

using ll = long long;
#define rng(i, l, r) for (int i = int(l); i < int(r); i++)
#define rep(i, r) rng(i, 0, r)
#define rrng(i, l, r) for (int i = int(r) - 1; i >= int(l); i--)
#define rrep(i, r) rrng(i, 0, r)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define one(x) memset(x, -1, sizeof(x))
#define zero(x) memset(x, 0, sizeof(x))
#define F first
#define S second
#define bg begin()
#define ed end()
#define all(x) x.bg, x.ed
#define si(x) int(x.size())
#define inf INT_MAX / 2 - 100
#define infl LLONG_MAX / 3
#ifdef LOCAL
#define dmp(x) cerr << __LINE__ << ' ' << #x << ' ' << x << endl
#else
#define dmp(x) void(0)
#endif

template <class t, class u> void chmax(t &a, u b) {
  if (a < b)
    a = b;
}
template <class t, class u> void chmin(t &a, u b) {
  if (b < a)
    a = b;
}

template <class t> using vc = vector<t>;
template <class t> using vvc = vector<vector<t>>;

using pi = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vc<int>;
using vl = vc<ll>;

ll readl(void) {
  ll x;
  cin >> x;
  return x;
}
int readi(void) {
  int x;
  cin >> x;
  return x;
}

string readstr() {
  string s;
  cin >> s;
  return s;
}
vi readvi(int n, int off = 0) {
  vi v(n);
  rep(i, n) v[i] = readi(), v[i] += off;
  return v;
}

vl readvl(int n, int off = 0) {
  vl v(n);
  rep(i, n) v[i] = readl(), v[i] += off;
  return v;
}

template <class t> void print(t x, int suc = 1) {
  cout << x;
  if (suc == 1)
    cout << "\n";
  if (suc == 2)
    cout << " ";
}

template <class t> void print(const vc<t> &v, int suc = 1) {
  rep(i, si(v)) print(v[i], i == int(si(v)) - 1 ? 1 : suc);
}

template <class t> bool inc(t a, t b, t c) { return !(c < b || b < a); }

template <class t> void compress(vc<t> &v) {
  sort(all(v));
  v.erase(unique(all(v)), v.ed);
}

template <class t> int lwb(const vc<t> &v, const t &a) {
  return lower_bound(all(v), a) - v.bg;
}

template <class t> struct Compress {
  vc<t> v;
  Compress() = default;
  Compress(const vc<t> &x) { add(x); }
  Compress(const initializer_list<vc<t>> &x) {
    for (auto &p : x)
      add(p);
  }
  void add(const t &x) { v.eb(x); }
  void add(const vc<t> &x) { copy(all(x), back_inserter(v)); }
  void build() { compress(v); }
  int get(const t &x) const { return lwb(v, x); }
  vc<t> get(const vc<t> &x) const {
    vc<t> res(x);
    for (auto &p : res)
      p = get(p);
    return res;
  }
  const t &operator[](int x) const { return v[x]; }
};
void Yes(bool ex = true) {
  cout << "Yes\n";
  if (ex)
    exit(0);
}
void YES(bool ex = true) {
  cout << "YES\n";
  if (ex)
    exit(0);
}
void No(bool ex = true) {
  cout << "No\n";
  if (ex)
    exit(0);
}
void NO(bool ex = true) {
  cout << "NO\n";
  if (ex)
    exit(0);
}
void orYes(bool x, bool ex = true) {
  if (x)
    Yes(ex);
  else
    No(ex);
}
void orYES(bool x, bool ex = true) {
  if (x)
    YES(ex);
  else
    NO(ex);
}
void Possible(bool ex = true) {
  cout << "Possible\n";
  if (ex)
    exit(0);
}
void POSSIBLE(bool ex = true) {
  cout << "POSSIBLE\n";
  if (ex)
    exit(0);
}
void Impossible(bool ex = true) {
  cout << "Impossible\n";
  if (ex)
    exit(0);
}
void IMPOSSIBLE(bool ex = true) {
  cout << "IMPOSSIBLE\n";
  if (ex)
    exit(0);
}
void orPossible(bool x, bool ex = true) {
  if (x)
    Possible(ex);
  else
    Impossible(ex);
}
void orPOSSIBLE(bool x, bool ex = true) {
  if (x)
    POSSIBLE(ex);
  else
    IMPOSSIBLE(ex);
}
using uint = unsigned;
using ull = unsigned long long;

template <uint const &MOD> struct Modular {
  static constexpr uint const &mod = MOD;
  uint v;
  Modular(long long x = 0) { c(x % mod + mod); }
  Modular &c(uint x) {
    v = x < mod ? x : x - mod;
    return *this;
  }
  Modular pow(int k) const {
    Modular res(1), tmp(v);
    while (k) {
      if (k & 1)
        res *= tmp;
      tmp *= tmp;
      k >>= 1;
    }
    return res;
  }

  Modular inv() const { return pow(mod - 2); }
  Modular operator-() const { return Modular(mod - v); }

  Modular &operator+=(const Modular &x) { return c(v + x.v); }
  Modular &operator-=(const Modular &x) { return c(v + mod - x.v); }
  Modular &operator*=(const Modular &x) {
    v = ull(v) * x.v % mod;
    return *this;
  }
  Modular &operator/=(const Modular &x) { return *this *= x.inv(); }

  Modular operator+(const Modular &x) const { return Modular(*this) += x; }
  Modular operator-(const Modular &x) const { return Modular(*this) -= x; }
  Modular operator*(const Modular &x) const { return Modular(*this) *= x; }
  Modular operator/(const Modular &x) const { return Modular(*this) /= x; }

  friend Modular operator+(long long x, const Modular &y) {
    return Modular(x) + y;
  }
  friend Modular operator-(long long x, const Modular &y) {
    return Modular(x) - y;
  }
  friend Modular operator*(long long x, const Modular &y) {
    return Modular(x) * y;
  }
  friend Modular operator/(long long x, const Modular &y) {
    return Modular(x) / y;
  }

  friend ostream &operator<<(ostream &os, const Modular &x) {
    return os << x.v;
  }
  friend istream &operator>>(istream &is, Modular &x) {
    long long p;
    is >> p;
    x = Modular(p);
    return is;
  }
  bool operator==(const Modular &x) const { return v == x.v; }
  bool operator!=(const Modular &x) const { return v != x.v; }
  bool operator<(const Modular &x) const { return v < x.v; }
  explicit operator bool() const { return v; }
};

vi g[200005];
int d[200005];
void dfs(int v, int p, int x) {
  d[v] = x;
  for (auto &i : g[v]) {
    if (i != p) {
      dfs(i, v, x + 1);
    }
  }
}
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(0);
  int n;
  cin >> n;
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    g[--a].pb(--b);
    g[b].pb(a);
  }
  dfs(0, -1, 0);
  pi ma = mp(0, 0);
  rep(i, n) { chmax(ma, mp(d[i], i)); }
  dfs(ma.S, -1, 0);
  ma = mp(0, ma.S);
  rep(i, n) { chmax(ma, mp(d[i], i)); }
  if ((ma.F + 1) % 3 == 2)
    print("Second");
  else
    print("First");
}