Problem p03055 (C++)

Buggy submission (status: Runtime Error):
// template version 1.15
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF = 1e18;

// define basic macro {{{
#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (int i = (int)((b)-1); i >= (int)(a); --i)
#define rrep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x)                                                                 \
  sort(all(x));                                                                \
  x.erase(unique(all(x)), x.end())
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x) - A.begin())
#define posu(A, x) (upper_bound(all(A), x) - A.begin())
template <class T> inline void chmax(T &a, const T &b) {
  if ((a) < (b))
    (a) = (b);
}
template <class T> inline void chmin(T &a, const T &b) {
  if ((a) > (b))
    (a) = (b);
}

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int, int> pii;
typedef tuple<int, int, int> iii;

template <typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

void sputs() {}
template <class Head, class... Tail> void sputs(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(Tail) == 0) {
    cout << endl;
  } else {
    cout << " ";
  }
  sputs(std::move(tail)...);
}

#if defined(PCM) || defined(LOCAL)
#include "lib/dump.hpp"
#else
#define dump(...) 42
#define dump_1d(...) 42
#define dump_2d(...) 42
#endif
//}}}

signed main() {
  int n;
  cin >> n;
  vvi g(n);
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].pb(b);
    g[b].pb(a);
  }

  int x = -1;
  int max_d = 0;
  auto dfs = [&](const auto &dfs, auto &d, int u, int cost) -> void {
    each(v, g[u]) {
      if (d[v] < INF)
        continue;
      d[v] = cost + 1;
      if (d[v] > max_d) {
        chmax(max_d, d[v]);
        x = v;
      }
      dfs(dfs, d, v, cost + 1);
    }
  };

  vi d0(n, INF);
  d0[0] = 0;
  dfs(dfs, d0, 0, 0);

  vi dx(n, INF);
  dx[x] = 0;
  max_d = 0;
  dfs(dfs, dx, x, 0);
  dump(max_d);

  vi dp(n + 1);

  dp[0] = 0;
  dp[1] = 1;
  dp[2] = 0;
  rep(i, 3, n + 1) {
    if (dp[i - 1] == 0 || dp[i - 2] == 0)
      dp[i] = 1;
  }
  dump(dp);
  cout << (dp[max_d + 1] ? "First" : "Second") << endl;

  return 0;
}


Fixed submission:
// template version 1.15
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF = 1e18;

// define basic macro {{{
#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (int i = (int)((b)-1); i >= (int)(a); --i)
#define rrep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x)                                                                 \
  sort(all(x));                                                                \
  x.erase(unique(all(x)), x.end())
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x) - A.begin())
#define posu(A, x) (upper_bound(all(A), x) - A.begin())
template <class T> inline void chmax(T &a, const T &b) {
  if ((a) < (b))
    (a) = (b);
}
template <class T> inline void chmin(T &a, const T &b) {
  if ((a) > (b))
    (a) = (b);
}

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int, int> pii;
typedef tuple<int, int, int> iii;

template <typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

void sputs() {}
template <class Head, class... Tail> void sputs(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(Tail) == 0) {
    cout << endl;
  } else {
    cout << " ";
  }
  sputs(std::move(tail)...);
}

#if defined(PCM) || defined(LOCAL)
#include "lib/dump.hpp"
#else
#define dump(...) 42
#define dump_1d(...) 42
#define dump_2d(...) 42
#endif
//}}}

signed main() {
  int n;
  cin >> n;
  if (n == 1) {
    cout << "First" << endl;
    return 0;
  }
  vvi g(n);
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].pb(b);
    g[b].pb(a);
  }

  int x = -1;
  int max_d = 0;
  auto dfs = [&](const auto &dfs, auto &d, int u, int cost) -> void {
    each(v, g[u]) {
      if (d[v] < INF)
        continue;
      d[v] = cost + 1;
      if (d[v] > max_d) {
        chmax(max_d, d[v]);
        x = v;
      }
      dfs(dfs, d, v, cost + 1);
    }
  };

  vi d0(n, INF);
  d0[0] = 0;
  dfs(dfs, d0, 0, 0);

  vi dx(n, INF);
  dx[x] = 0;
  max_d = 0;
  dfs(dfs, dx, x, 0);
  dump(max_d);

  vi dp(n + 1);

  dp[0] = 0;
  dp[1] = 1;
  dp[2] = 0;
  rep(i, 3, n + 1) {
    if (dp[i - 1] == 0 || dp[i - 2] == 0)
      dp[i] = 1;
  }
  dump(dp);
  cout << (dp[max_d + 1] ? "First" : "Second") << endl;

  return 0;
}