Problem p03055 (C++)

Buggy submission (status: Runtime Error):
#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <valarray>
#include <vector>

using namespace std;
using uint = uint32_t;
using LL = int64_t;
using ULL = uint64_t;
using PP = pair<LL, LL>;
template <typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T>>;
#define REP(i, a, n) for (LL i = (a), i##_max_ = (n); i < i##_max_; ++i)
#define REM(i, a, n) for (LL i = (LL)(n)-1, i##_min_ = (a); i >= i##_min_; --i)
#define FLOAT fixed << setprecision(16)
#define SPEEDUP                                                                \
  {                                                                            \
    cin.tie(NULL);                                                             \
    ios::sync_with_stdio(false);                                               \
  }
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream &operator>>(istream &ist, pair<T, U> &right) {
  return ist >> right.first >> right.second;
}
template <class T, class U>
ostream &operator<<(ostream &ost, const pair<T, U> &right) {
  return ost << right.first << ' ' << right.second;
}
template <class T, class TCompatible, size_t N>
void Fill(T (&dest)[N], const TCompatible &val) {
  fill(dest, dest + N, val);
}
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T (&dest)[M][N], const TCompatible &val) {
  for (int i = 0; i < M; ++i)
    Fill(dest[i], val);
}
template <class T> T Next() {
  T buf;
  cin >> buf;
  return buf;
}
istream &Ignore(istream &ist) {
  string s;
  ist >> s;
  return ist;
}
bool Inside(int i, int j, int h, int w) {
  return i >= 0 && i < h && j >= 0 && j < w;
}

#ifdef ONLY_MY_ENVIR
#include "Accumulator.h"
#include "Algebraic.h"
#include "BIT.h"
#include "BinaryMatrix.h"
#include "BinaryTree.h"
#include "Bipartite.h"
#include "Compressor.h"
#include "Decompositions.h"
#include "DiscreteLog.h"
#include "DynamicMod.h"
#include "Exponential.h"
#include "FFT.h"
#include "Factorization.h"
#include "FlowSolver.h"
#include "Geometric2D.h"
#include "Geometric2DFloat.h"
#include "Geometric3D.h"
#include "Geometric3DFloat.h"
#include "Graph.h"
#include "GraphUtil.h"
#include "IntMod.h"
#include "Interpolation.h"
#include "KDTree.h"
#include "LIS.h"
#include "LazySegmentTree.h"
#include "List.h"
#include "Math.h"
#include "MathUtil.h"
#include "Matrix.h"
#include "MinCostFlowSolver.h"
#include "MinMax.h"
#include "Numbers.h"
#include "Optimize.h"
#include "Permutation.h"
#include "Polynomial.h"
#include "Position.h"
#include "Random.h"
#include "Range.h"
#include "Rational.h"
#include "RuntimeMod.h"
#include "SegmentTree.h"
#include "SegmentTree2D.h"
#include "Sets.h"
#include "Shortest.h"
#include "SlidingWindow.h"
#include "SpanningTree.h"
#include "StringSearching.h"
#include "SuffixArray.h"
#include "SwitchList.h"
#include "Timer.h"
#include "Tree.h"
#include "TreeUtil.h"
#include "UnionFind.h"
#include "Util.h"
#include "VectorUtil.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream &operator>>(istream &ist, __int128 &val) {
  LL tmp;
  ist >> tmp;
  val = tmp;
  return ist;
}
ostream &operator<<(ostream &ost, __int128 val) {
  LL tmp = val;
  ost << tmp;
  return ost;
}
#endif

int n;
vector<int> G[100000];
int D[100000];
void rec(int from, int prev, int dep) {
  D[from] = dep;
  for (int to : G[from]) {
    if (to == prev)
      continue;
    rec(to, from, dep + 1);
  }
}

int sub(int from) {
  Fill(D, 0);
  rec(from, -1, 0);
  return max_element(D, D + n) - D;
}

int main() {
  cin >> n;
  REP(i, 1, n) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  int v = sub(0);
  int u = sub(v);
  int d = D[u];
  cout << (d % 3 == 1 ? "Second" : "First") << endl;
  return 0;
}


Fixed submission:
#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <valarray>
#include <vector>

using namespace std;
using uint = uint32_t;
using LL = int64_t;
using ULL = uint64_t;
using PP = pair<LL, LL>;
template <typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T>>;
#define REP(i, a, n) for (LL i = (a), i##_max_ = (n); i < i##_max_; ++i)
#define REM(i, a, n) for (LL i = (LL)(n)-1, i##_min_ = (a); i >= i##_min_; --i)
#define FLOAT fixed << setprecision(16)
#define SPEEDUP                                                                \
  {                                                                            \
    cin.tie(NULL);                                                             \
    ios::sync_with_stdio(false);                                               \
  }
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream &operator>>(istream &ist, pair<T, U> &right) {
  return ist >> right.first >> right.second;
}
template <class T, class U>
ostream &operator<<(ostream &ost, const pair<T, U> &right) {
  return ost << right.first << ' ' << right.second;
}
template <class T, class TCompatible, size_t N>
void Fill(T (&dest)[N], const TCompatible &val) {
  fill(dest, dest + N, val);
}
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T (&dest)[M][N], const TCompatible &val) {
  for (int i = 0; i < M; ++i)
    Fill(dest[i], val);
}
template <class T> T Next() {
  T buf;
  cin >> buf;
  return buf;
}
istream &Ignore(istream &ist) {
  string s;
  ist >> s;
  return ist;
}
bool Inside(int i, int j, int h, int w) {
  return i >= 0 && i < h && j >= 0 && j < w;
}

#ifdef ONLY_MY_ENVIR
#include "Accumulator.h"
#include "Algebraic.h"
#include "BIT.h"
#include "BinaryMatrix.h"
#include "BinaryTree.h"
#include "Bipartite.h"
#include "Compressor.h"
#include "Decompositions.h"
#include "DiscreteLog.h"
#include "DynamicMod.h"
#include "Exponential.h"
#include "FFT.h"
#include "Factorization.h"
#include "FlowSolver.h"
#include "Geometric2D.h"
#include "Geometric2DFloat.h"
#include "Geometric3D.h"
#include "Geometric3DFloat.h"
#include "Graph.h"
#include "GraphUtil.h"
#include "IntMod.h"
#include "Interpolation.h"
#include "KDTree.h"
#include "LIS.h"
#include "LazySegmentTree.h"
#include "List.h"
#include "Math.h"
#include "MathUtil.h"
#include "Matrix.h"
#include "MinCostFlowSolver.h"
#include "MinMax.h"
#include "Numbers.h"
#include "Optimize.h"
#include "Permutation.h"
#include "Polynomial.h"
#include "Position.h"
#include "Random.h"
#include "Range.h"
#include "Rational.h"
#include "RuntimeMod.h"
#include "SegmentTree.h"
#include "SegmentTree2D.h"
#include "Sets.h"
#include "Shortest.h"
#include "SlidingWindow.h"
#include "SpanningTree.h"
#include "StringSearching.h"
#include "SuffixArray.h"
#include "SwitchList.h"
#include "Timer.h"
#include "Tree.h"
#include "TreeUtil.h"
#include "UnionFind.h"
#include "Util.h"
#include "VectorUtil.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream &operator>>(istream &ist, __int128 &val) {
  LL tmp;
  ist >> tmp;
  val = tmp;
  return ist;
}
ostream &operator<<(ostream &ost, __int128 val) {
  LL tmp = val;
  ost << tmp;
  return ost;
}
#endif

int n;
vector<int> G[200000];
int D[200000];
void rec(int from, int prev, int dep) {
  D[from] = dep;
  for (int to : G[from]) {
    if (to == prev)
      continue;
    rec(to, from, dep + 1);
  }
}

int sub(int from) {
  Fill(D, 0);
  rec(from, -1, 0);
  return max_element(D, D + n) - D;
}

int main() {
  cin >> n;
  REP(i, 1, n) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  int v = sub(0);
  int u = sub(v);
  int d = D[u];
  cout << (d % 3 == 1 ? "Second" : "First") << endl;
  return 0;
}