Problem p03055 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <forward_list>
#include <initializer_list>
#include <tuple>

#define cauto const auto &
#else

#endif

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

typedef vector<int> vint;
typedef vector<vector<int>> vvint;
typedef vector<long long> vll;
typedef vector<vector<long long>> vvll;

#define VV(T) vector<vector<T>>

template <class T>
void initvv(vector<vector<T>> &v, int a, int b, const T &t = T()) {
  v.assign(a, vector<T>(b, t));
}

template <class F, class T> void convert(const F &f, T &t) {
  stringstream ss;
  ss << f;
  ss >> t;
}

#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2((i), 0, (n))
#define _rep2(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)
#define ALL(v) (v).begin(), (v).end()
#define PB push_back
#define fi first
#define se second
#define mkp make_pair
#define DEBUG
#ifdef DEBUG
#define dump(x) cout << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
#else
#define dump(x)
#define debug(x)
#endif

#define MOD 1000000007LL
#define EPS 1e-8
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
#define maxs(x, y) x = max(x, y)
#define mins(x, y) x = min(x, y)

vvint vv;

void dfs(int ind, int d, int pre, vint &vis) {
  if (vis[ind] != -1) {
    return;
  }
  vis[ind] = d;
  for (int x : vv[ind]) {
    if (x == pre) {
      continue;
    }
    dfs(x, d + 1, ind, vis);
  }
}

void main2() {
  int N;
  cin >> N;
  vv = vvint(N);
  rep(i, N) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    vv[a].PB(b);
    swap(a, b);
    vv[a].PB(b);
  }
  vint vis(N, -1);
  dfs(0, 0, -1, vis);
  int ind = 0;
  rep(i, N) {
    if (vis[ind] < vis[i]) {
      ind = i;
    }
  }
  vis = vint(N, -1);
  dfs(ind, 0, -1, vis);
  int ans = 0;
  rep(i, N) { maxs(ans, vis[i]); }
  if (ans % 3 == 1) {
    cout << "Second" << endl;
  } else {
    cout << "First" << endl;
  }
}

signed main() { main2(); }

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <forward_list>
#include <initializer_list>
#include <tuple>

#define cauto const auto &
#else

#endif

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

typedef vector<int> vint;
typedef vector<vector<int>> vvint;
typedef vector<long long> vll;
typedef vector<vector<long long>> vvll;

#define VV(T) vector<vector<T>>

template <class T>
void initvv(vector<vector<T>> &v, int a, int b, const T &t = T()) {
  v.assign(a, vector<T>(b, t));
}

template <class F, class T> void convert(const F &f, T &t) {
  stringstream ss;
  ss << f;
  ss >> t;
}

#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2((i), 0, (n))
#define _rep2(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)
#define ALL(v) (v).begin(), (v).end()
#define PB push_back
#define fi first
#define se second
#define mkp make_pair
#define DEBUG
#ifdef DEBUG
#define dump(x) cout << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
#else
#define dump(x)
#define debug(x)
#endif

#define MOD 1000000007LL
#define EPS 1e-8
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
#define maxs(x, y) x = max(x, y)
#define mins(x, y) x = min(x, y)

vvint vv;

void dfs(int ind, int d, int pre, vint &vis) {
  if (vis[ind] != -1) {
    return;
  }
  vis[ind] = d;
  for (int x : vv[ind]) {
    if (x == pre) {
      continue;
    }
    dfs(x, d + 1, ind, vis);
  }
}

void main2() {
  int N;
  cin >> N;
  vv = vvint(N);
  rep(i, N - 1) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    vv[a].PB(b);
    swap(a, b);
    vv[a].PB(b);
  }
  vint vis(N, -1);
  dfs(0, 0, -1, vis);
  int ind = 0;
  rep(i, N) {
    if (vis[ind] < vis[i]) {
      ind = i;
    }
  }
  vis = vint(N, -1);
  dfs(ind, 0, -1, vis);
  int ans = 0;
  rep(i, N) { maxs(ans, vis[i]); }
  if (ans % 3 == 1) {
    cout << "Second" << endl;
  } else {
    cout << "First" << endl;
  }
}

signed main() { main2(); }