Problem p03055 (C++)

Buggy submission (status: Runtime Error):
/*
Author: QAQAutomaton
Lang: C++
Code: C.cpp
Mail: lk@qaq-am.com
Blog: https://www.qaq-am.com/
*/
#include <bits/stdc++.h>
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define DEBUG printf("Passing [%s] in LINE %d\n", __FUNCTION__, __LINE__)
#define Debug debug("Passing [%s] in LINE %d\n", __FUNCTION__, __LINE__)
#define all(x) x.begin(), x.end()
#define x first
#define y second
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef complex<double> cp;
typedef pair<int, int> pii;
int inf;
const double eps = 1e-8;
const double pi = acos(-1.0);
template <class T, class T2> int chkmin(T &a, T2 b) {
  return a > b ? a = b, 1 : 0;
}
template <class T, class T2> int chkmax(T &a, T2 b) {
  return a < b ? a = b, 1 : 0;
}
template <class T> T sqr(T a) { return a * a; }
template <class T, class T2> T mmin(T a, T2 b) { return a < b ? a : b; }
template <class T, class T2> T mmax(T a, T2 b) { return a > b ? a : b; }
template <class T> T aabs(T a) { return a < 0 ? -a : a; }
template <class T> int dcmp(T a, T b) { return a > b; }
template <int *a> int cmp_a(int x, int y) { return a[x] < a[y]; }
#define min mmin
#define max mmax
#define abs aabs
struct __INIT__ {
  __INIT__() { memset(&inf, 0x3f, sizeof(inf)); }
} __INIT___;
namespace io {
const int SIZE = (1 << 21) + 1;
char ibuf[SIZE], *iS, *iT, obuf[SIZE], *oS = obuf, *oT = oS + SIZE - 1, c,
                                       qu[55];
int f, qr;
// getchar
#define gc()                                                                   \
  (iS == iT ? (iT = (iS = ibuf) + fread(ibuf, 1, SIZE, stdin),                 \
               (iS == iT ? EOF : *iS++))                                       \
            : *iS++)
// print the remaining part
inline void flush() {
  fwrite(obuf, 1, oS - obuf, stdout);
  oS = obuf;
}
// putchar
inline void putc(char x) {
  *oS++ = x;
  if (oS == oT)
    flush();
}
template <typename A> inline bool read(A &x) {
  for (f = 1, c = gc(); c < '0' || c > '9'; c = gc())
    if (c == '-')
      f = -1;
    else if (c == EOF)
      return 0;
  for (x = 0; c <= '9' && c >= '0'; c = gc())
    x = x * 10 + (c & 15);
  x *= f;
  return 1;
}
inline bool read(char &x) {
  while ((x = gc()) == ' ' || x == '\n' || x == '\r')
    ;
  return x != EOF;
}
inline bool read(char *x) {
  while ((*x = gc()) == '\n' || *x == ' ' || *x == '\r')
    ;
  if (*x == EOF)
    return 0;
  while (!(*x == '\n' || *x == ' ' || *x == '\r' || *x == EOF))
    *(++x) = gc();
  *x = 0;
  return 1;
}
template <typename A, typename... B> inline bool read(A &x, B &...y) {
  return read(x) && read(y...);
}
template <typename A> inline bool write(A x) {
  if (!x)
    putc('0');
  if (x < 0)
    putc('-'), x = -x;
  while (x)
    qu[++qr] = x % 10 + '0', x /= 10;
  while (qr)
    putc(qu[qr--]);
  return 0;
}
inline bool write(char x) {
  putc(x);
  return 0;
}
inline bool write(const char *x) {
  while (*x) {
    putc(*x);
    ++x;
  }
  return 0;
}
inline bool write(char *x) {
  while (*x) {
    putc(*x);
    ++x;
  }
  return 0;
}
template <typename A, typename... B> inline bool write(A x, B... y) {
  return write(x) || write(y...);
}
// no need to call flush at the end manually!
struct Flusher_ {
  ~Flusher_() { flush(); }
} io_flusher_;
} // namespace io
using io ::putc;
using io ::read;
using io ::write;
vector<int> to[100005];
int q[100005], d[100005], *l, *r;
int f[100005];
int calc(int n) {
  f[1] = 1;
  f[2] = 0;
  for (int i = 3; i <= n; ++i) {
    f[i] = !(f[i - 1] && f[i - 2]);
  }
  return f[n];
}
signed main() {
#ifdef QAQAutoMaton
  freopen("C.in", "r", stdin);
  freopen("C.out", "w", stdout);
#endif
  int n, u, v;
  read(n);
  for (int i = 1; i < n; ++i) {
    read(u, v);
    to[u].push_back(v);
    to[v].push_back(u);
  }
  d[1] = 1;
  *(l = r = q) = 1;
  while (l <= r) {
    for (auto i : to[*l])
      if (!d[i]) {
        d[i] = d[*l] + 1;
        *(++r) = i;
      }
    ++l;
  }
  int x = *r;
  for (int i = 1; i <= n; ++i)
    d[i] = 0;
  d[x] = 1;
  *(l = r = q) = x;
  while (l <= r) {
    for (auto i : to[*l])
      if (!d[i]) {
        d[i] = d[*l] + 1;
        *(++r) = i;
      }
    ++l;
  }
  write(calc(d[*r]) ? "First\n" : "Second\n");
  return 0;
}


Fixed submission:
/*
Author: QAQAutomaton
Lang: C++
Code: C.cpp
Mail: lk@qaq-am.com
Blog: https://www.qaq-am.com/
*/
#include <bits/stdc++.h>
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define DEBUG printf("Passing [%s] in LINE %d\n", __FUNCTION__, __LINE__)
#define Debug debug("Passing [%s] in LINE %d\n", __FUNCTION__, __LINE__)
#define all(x) x.begin(), x.end()
#define x first
#define y second
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef complex<double> cp;
typedef pair<int, int> pii;
int inf;
const double eps = 1e-8;
const double pi = acos(-1.0);
template <class T, class T2> int chkmin(T &a, T2 b) {
  return a > b ? a = b, 1 : 0;
}
template <class T, class T2> int chkmax(T &a, T2 b) {
  return a < b ? a = b, 1 : 0;
}
template <class T> T sqr(T a) { return a * a; }
template <class T, class T2> T mmin(T a, T2 b) { return a < b ? a : b; }
template <class T, class T2> T mmax(T a, T2 b) { return a > b ? a : b; }
template <class T> T aabs(T a) { return a < 0 ? -a : a; }
template <class T> int dcmp(T a, T b) { return a > b; }
template <int *a> int cmp_a(int x, int y) { return a[x] < a[y]; }
#define min mmin
#define max mmax
#define abs aabs
struct __INIT__ {
  __INIT__() { memset(&inf, 0x3f, sizeof(inf)); }
} __INIT___;
namespace io {
const int SIZE = (1 << 21) + 1;
char ibuf[SIZE], *iS, *iT, obuf[SIZE], *oS = obuf, *oT = oS + SIZE - 1, c,
                                       qu[55];
int f, qr;
// getchar
#define gc()                                                                   \
  (iS == iT ? (iT = (iS = ibuf) + fread(ibuf, 1, SIZE, stdin),                 \
               (iS == iT ? EOF : *iS++))                                       \
            : *iS++)
// print the remaining part
inline void flush() {
  fwrite(obuf, 1, oS - obuf, stdout);
  oS = obuf;
}
// putchar
inline void putc(char x) {
  *oS++ = x;
  if (oS == oT)
    flush();
}
template <typename A> inline bool read(A &x) {
  for (f = 1, c = gc(); c < '0' || c > '9'; c = gc())
    if (c == '-')
      f = -1;
    else if (c == EOF)
      return 0;
  for (x = 0; c <= '9' && c >= '0'; c = gc())
    x = x * 10 + (c & 15);
  x *= f;
  return 1;
}
inline bool read(char &x) {
  while ((x = gc()) == ' ' || x == '\n' || x == '\r')
    ;
  return x != EOF;
}
inline bool read(char *x) {
  while ((*x = gc()) == '\n' || *x == ' ' || *x == '\r')
    ;
  if (*x == EOF)
    return 0;
  while (!(*x == '\n' || *x == ' ' || *x == '\r' || *x == EOF))
    *(++x) = gc();
  *x = 0;
  return 1;
}
template <typename A, typename... B> inline bool read(A &x, B &...y) {
  return read(x) && read(y...);
}
template <typename A> inline bool write(A x) {
  if (!x)
    putc('0');
  if (x < 0)
    putc('-'), x = -x;
  while (x)
    qu[++qr] = x % 10 + '0', x /= 10;
  while (qr)
    putc(qu[qr--]);
  return 0;
}
inline bool write(char x) {
  putc(x);
  return 0;
}
inline bool write(const char *x) {
  while (*x) {
    putc(*x);
    ++x;
  }
  return 0;
}
inline bool write(char *x) {
  while (*x) {
    putc(*x);
    ++x;
  }
  return 0;
}
template <typename A, typename... B> inline bool write(A x, B... y) {
  return write(x) || write(y...);
}
// no need to call flush at the end manually!
struct Flusher_ {
  ~Flusher_() { flush(); }
} io_flusher_;
} // namespace io
using io ::putc;
using io ::read;
using io ::write;
vector<int> to[200005];
int q[200005], d[200005], *l, *r;
int f[200005];
int calc(int n) {
  f[1] = 1;
  f[2] = 0;
  for (int i = 3; i <= n; ++i) {
    f[i] = !(f[i - 1] && f[i - 2]);
  }
  return f[n];
}
signed main() {
#ifdef QAQAutoMaton
  freopen("C.in", "r", stdin);
  freopen("C.out", "w", stdout);
#endif
  int n, u, v;
  read(n);
  for (int i = 1; i < n; ++i) {
    read(u, v);
    to[u].push_back(v);
    to[v].push_back(u);
  }
  d[1] = 1;
  *(l = r = q) = 1;
  while (l <= r) {
    for (auto i : to[*l])
      if (!d[i]) {
        d[i] = d[*l] + 1;
        *(++r) = i;
      }
    ++l;
  }
  int x = *r;
  for (int i = 1; i <= n; ++i)
    d[i] = 0;
  d[x] = 1;
  *(l = r = q) = x;
  while (l <= r) {
    for (auto i : to[*l])
      if (!d[i]) {
        d[i] = d[*l] + 1;
        *(++r) = i;
      }
    ++l;
  }
  write(calc(d[*r]) ? "First\n" : "Second\n");
  return 0;
}