Problem p02834 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define fast_io                                                                \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define MOD (long long int)(1e9 + 7)
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  ret *= kaijo(n);
  ret %= MOD;
  ret *= gyaku_kaijo(r);
  ret %= MOD;
  ret *= gyaku_kaijo(n - r);
  ret %= MOD;
  return ret;
}

vector<int> G[(int)(1e5 + 3)];
bool oni[(int)(1e5 + 3)], nige[(int)(1e5 + 3)];
bool checkOni[(int)(1e5 + 3)], checkNige[(int)(1e5 + 3)];

int main(void) {
  fast_io int u, v;
  int n;
  cin >> n >> u >> v;
  u--;
  v--;
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  rep(i, n) {
    checkOni[i] = false;
    checkNige[i] = false;
    oni[i] = false;
    nige[i] = false;
  }
  queue<int> A, B;
  A.push(u);
  B.push(v);
  oni[v] = true;
  ll countNige = 0;
  if (G[u].size() >= 2) {
    countNige++;
    nige[u] = true;
  }
  ll ans = 0;
  while (true) {
    queue<int> neA, neB;
    while (A.size() > 0) {
      int a = A.front();
      A.pop();
      if (!nige[a] && a != u) {
        continue;
      }
      rep(j, G[a].size()) {
        int next = G[a][j];
        if (!checkNige[next] && !oni[next] && G[next].size() >= 2) {
          checkNige[next] = true;
          nige[next] = true;
          countNige++;
          neA.push(next);
        }
      }
    }
    if (countNige == 0) {
      break;
    }
    ans++;
    while (B.size() > 0) {
      int b = B.front();
      B.pop();
      rep(j, G[b].size()) {
        int next = G[b][j];
        if (!checkOni[next]) {
          checkOni[next] = true;
          oni[next] = true;
          if (nige[next]) {
            nige[next] = false;
            countNige--;
          }
          neB.push(next);
        }
      }
    }
    A = neA;
    B = neB;
    if (countNige == 0) {
      break;
    }
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define fast_io                                                                \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define MOD (long long int)(1e9 + 7)
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  ret *= kaijo(n);
  ret %= MOD;
  ret *= gyaku_kaijo(r);
  ret %= MOD;
  ret *= gyaku_kaijo(n - r);
  ret %= MOD;
  return ret;
}

vector<int> G[(int)(1e5 + 3)];
bool oni[(int)(1e5 + 3)], nige[(int)(1e5 + 3)];
bool checkOni[(int)(1e5 + 3)], checkNige[(int)(1e5 + 3)];

int main(void) {
  fast_io int u, v;
  int n;
  cin >> n >> u >> v;
  u--;
  v--;
  rep(i, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  rep(i, n) {
    checkOni[i] = false;
    checkNige[i] = false;
    oni[i] = false;
    nige[i] = false;
  }
  queue<int> A, B;
  A.push(u);
  B.push(v);
  oni[v] = true;
  checkOni[v] = true;
  checkNige[u] = true;
  ll countNige = 0;
  if (G[u].size() >= 2) {
    countNige++;
    nige[u] = true;
  }
  ll ans = 0;
  while (true) {
    queue<int> neA, neB;
    while (A.size() > 0) {
      int a = A.front();
      A.pop();
      if (!nige[a] && a != u) {
        continue;
      }
      rep(j, G[a].size()) {
        int next = G[a][j];
        if (!checkNige[next] && !oni[next] && G[next].size() >= 2) {
          checkNige[next] = true;
          nige[next] = true;
          countNige++;
          neA.push(next);
        }
      }
    }
    if (countNige == 0) {
      break;
    }
    ans++;
    while (B.size() > 0) {
      int b = B.front();
      B.pop();
      rep(j, G[b].size()) {
        int next = G[b][j];
        if (!checkOni[next]) {
          checkOni[next] = true;
          oni[next] = true;
          if (nige[next]) {
            nige[next] = false;
            countNige--;
          }
          neB.push(next);
        }
      }
    }
    A = neA;
    B = neB;
    if (countNige == 0) {
      break;
    }
  }
  cout << ans << endl;
  return 0;
}