Problem p02834 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <utility>
#include <vector>

#define fir first
#define sec second
#define sz(s) (s).size();
#define pb push_back
#define get(n) scanf("%d", &n);
#define gets(s)                                                                \
  string s;                                                                    \
  cin >> (s);
#define prfi(n) printf("%d", &n);
#define prfd(n) printf("%lf", &n);
#define All(s) (s).begin(), (s).end()
#define rep(i, j, k) for (int(i) = (j); (i) <= (k); (i)++)
#define rep0(i, j) for (int(i) = 0; (i) < (j); (i)++)
#define repdown(i, j, k) for (int(i) = (j); (i) >= (k); (i)--)
#define repdown0(i, j) for (int(i) = (j); i >= 0; i--)
#define vfor(c, v) for (auto(c) : v)
#define dump(x) std::cout << #x << " = " << (x) << std::endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

using ll = long long;
using pii = std::pair<int, int>;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vd = std::vector<double>;
using vvd = std::vector<vd>;
using qi = std::queue<int>;
using vpii = std::vector<std::pair<int, int>>;
using namespace std;

const int Mod = (1e9) + 7;
const int max_n = 3 * (1e5) + 1;
const int max_m = 83 * (1e5) + 1;
const int INF = 10241024;
const ll INFL = (1ll << 62) - 1;
const int dx[] = {-1, 0, 0, 1};
const int dy[] = {0, -1, 1, 0};

//_____________________________________Templates_________________________________________//

template <class T1, class T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <class T1, class T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

// mainly use for dynamic prog
template <class T1, class T2> void update(T1 &a, T2 b) {
  a += b;
  if (a > Mod)
    a %= Mod;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

inline void OUT(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest) {
  cout << first << " ";
  OUT(rest...);
  return;
}

template <class T> struct segt {

  int n;
  T def;
  vector<T> dat;
  function<T(T, T)> operation;
  function<T(T, T)> update;

  segt(size_t _n, T _def, function<T(T, T)> _operation,
       function<T(T, T)> _update)
      : def(_def), operation(_operation), update(_update) {
    n = 1;
    while (n < _n) {
      n *= 2;
    }
    dat = vector<T>(2 * n - 1, _def);
  }

  T _query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return def;
    if (a <= r && l <= b)
      return dat[k];
    else {
      T vl = _query(a, b, k * 2 + 1, l, (l + r) / 2);
      T vr = _query(a, b, k * 2 + 2, (l + r) / 2, r);
      return operation(vl, vr);
    }
  }

  T query(int a, int b) { return _query(a, b, 0, 0, n); }

  void change(int i, T value) {
    i += n - 1;
    dat[i] = update(dat[i], value);
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] = operation(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }

  T operator[](int i) { return dat[i + n - 1]; }
};
//_____________________array calc____________________________________//
/*
vvi mul(vvi &A, vvi &B){
  vvi C(A.size(), vi(B.size()))
  rep(i,A.size()){
    rep(j,B.size()){
      rep(k,B[0].size()){
        C[i][j] = (C[i][j] A[i][k] + B[k][j]) % Mod;
      }
    }
  }

  return C;
}

vvi pow(vvi A, ll n){
  vvi B(A.size(), vi(A.size()));
  rep(i=0;i<A.size();i++){
    B[i][i] = 1;
  }

  while (n >0){
    if (n & 1) B = mul(B, A);
    A = mul(A, A);
    n = n >> 1;
  }
  return B;
}
*/
//_____________________Bynary Indexed Tree __________________________//
/*
struct BIT{
  #define bs (1 << 20)
  int bit[bs];

  int sum (int i){
    int s = 0;
    while(i > 0){
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }

  void add(int i, int x){
    while(i <= n){
      bit[i] += x;
      i += i & -i;
    }
  }
}
*/

//____________________RMQ____________________________//
/*struct RMQ{
  #define s (1 << 20)
  ll dat[s];
  int n;

  void init(){
    memset(dat, -1e10, sizeof(dat));
  }

  void upd(int k, ll a){
    k += s/2 - 1;
    dat[k] = a;
    while(k > 0){
      k = (k -1) /2;
      dat[k] = max(dat[k*2+1], dat[k*2+2]);
    }
  }

  ll query(int a, int b, int k, int l, int r){
    if(r <= a || b <= l) return -1e8;
    if( a <= l && r <= b)return dat[k];
    else
    {
      ll vl = query(a, b, k*2+1, l, (l+r)/2);
      ll vr = query(a, b, k*2+2, (l+r)/2, r);
      return max(vl, vr);
    }
  }

  void Debug(int a){
    a += s/2 - 1;
    rep(i,s/2 - 1,a){
      dump(dat[i]);
    }
  }
};
*/
//_____________________　following sorce code_________________________//
int n, m, k, x, y;
// vvll dp(400, vll(400));
vll dp(1 << 16, -1e18);
string s;
vvll v(16, vll(16));
vll cost(1 << 16);

vi dist;
vi dat[10005];
void dfs(int v, int u = -1, int score = 0) {
  dist[v] = score;
  for (auto c : dat[v]) {
    if (c == u)
      continue;
    dfs(c, v, score + 1);
  }
}
vi f(int x) {
  dist = vi(n);
  dfs(x);
  return dist;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int u, v;
  IN(n, u, v);
  u--;
  v--;
  rep0(i, n - 1) {
    int a, b;
    IN(a, b);
    a--;
    b--;
    dat[a].pb(b);
    dat[b].pb(a);
  }
  vi distU = f(u);
  vi distV = f(v);

  int mx = 0;
  rep0(i, n) {
    if (distV[i] > distU[i]) {
      chmax(mx, distV[i]);
    }
  }
  mx--;
  OUT(mx);
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <utility>
#include <vector>

#define fir first
#define sec second
#define sz(s) (s).size();
#define pb push_back
#define get(n) scanf("%d", &n);
#define gets(s)                                                                \
  string s;                                                                    \
  cin >> (s);
#define prfi(n) printf("%d", &n);
#define prfd(n) printf("%lf", &n);
#define All(s) (s).begin(), (s).end()
#define rep(i, j, k) for (int(i) = (j); (i) <= (k); (i)++)
#define rep0(i, j) for (int(i) = 0; (i) < (j); (i)++)
#define repdown(i, j, k) for (int(i) = (j); (i) >= (k); (i)--)
#define repdown0(i, j) for (int(i) = (j); i >= 0; i--)
#define vfor(c, v) for (auto(c) : v)
#define dump(x) std::cout << #x << " = " << (x) << std::endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

using ll = long long;
using pii = std::pair<int, int>;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vd = std::vector<double>;
using vvd = std::vector<vd>;
using qi = std::queue<int>;
using vpii = std::vector<std::pair<int, int>>;
using namespace std;

const int Mod = (1e9) + 7;
const int max_n = 3 * (1e5) + 1;
const int max_m = 83 * (1e5) + 1;
const int INF = 10241024;
const ll INFL = (1ll << 62) - 1;
const int dx[] = {-1, 0, 0, 1};
const int dy[] = {0, -1, 1, 0};

//_____________________________________Templates_________________________________________//

template <class T1, class T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <class T1, class T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

// mainly use for dynamic prog
template <class T1, class T2> void update(T1 &a, T2 b) {
  a += b;
  if (a > Mod)
    a %= Mod;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

inline void OUT(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest) {
  cout << first << " ";
  OUT(rest...);
  return;
}

template <class T> struct segt {

  int n;
  T def;
  vector<T> dat;
  function<T(T, T)> operation;
  function<T(T, T)> update;

  segt(size_t _n, T _def, function<T(T, T)> _operation,
       function<T(T, T)> _update)
      : def(_def), operation(_operation), update(_update) {
    n = 1;
    while (n < _n) {
      n *= 2;
    }
    dat = vector<T>(2 * n - 1, _def);
  }

  T _query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return def;
    if (a <= r && l <= b)
      return dat[k];
    else {
      T vl = _query(a, b, k * 2 + 1, l, (l + r) / 2);
      T vr = _query(a, b, k * 2 + 2, (l + r) / 2, r);
      return operation(vl, vr);
    }
  }

  T query(int a, int b) { return _query(a, b, 0, 0, n); }

  void change(int i, T value) {
    i += n - 1;
    dat[i] = update(dat[i], value);
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] = operation(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }

  T operator[](int i) { return dat[i + n - 1]; }
};
//_____________________array calc____________________________________//
/*
vvi mul(vvi &A, vvi &B){
  vvi C(A.size(), vi(B.size()))
  rep(i,A.size()){
    rep(j,B.size()){
      rep(k,B[0].size()){
        C[i][j] = (C[i][j] A[i][k] + B[k][j]) % Mod;
      }
    }
  }

  return C;
}

vvi pow(vvi A, ll n){
  vvi B(A.size(), vi(A.size()));
  rep(i=0;i<A.size();i++){
    B[i][i] = 1;
  }

  while (n >0){
    if (n & 1) B = mul(B, A);
    A = mul(A, A);
    n = n >> 1;
  }
  return B;
}
*/
//_____________________Bynary Indexed Tree __________________________//
/*
struct BIT{
  #define bs (1 << 20)
  int bit[bs];

  int sum (int i){
    int s = 0;
    while(i > 0){
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }

  void add(int i, int x){
    while(i <= n){
      bit[i] += x;
      i += i & -i;
    }
  }
}
*/

//____________________RMQ____________________________//
/*struct RMQ{
  #define s (1 << 20)
  ll dat[s];
  int n;

  void init(){
    memset(dat, -1e10, sizeof(dat));
  }

  void upd(int k, ll a){
    k += s/2 - 1;
    dat[k] = a;
    while(k > 0){
      k = (k -1) /2;
      dat[k] = max(dat[k*2+1], dat[k*2+2]);
    }
  }

  ll query(int a, int b, int k, int l, int r){
    if(r <= a || b <= l) return -1e8;
    if( a <= l && r <= b)return dat[k];
    else
    {
      ll vl = query(a, b, k*2+1, l, (l+r)/2);
      ll vr = query(a, b, k*2+2, (l+r)/2, r);
      return max(vl, vr);
    }
  }

  void Debug(int a){
    a += s/2 - 1;
    rep(i,s/2 - 1,a){
      dump(dat[i]);
    }
  }
};
*/
//_____________________　following sorce code_________________________//
int n, m, k, x, y;
// vvll dp(400, vll(400));
vll dp(1 << 16, -1e18);
string s;
vvll v(16, vll(16));
vll cost(1 << 16);

vi dist;
vi dat[100005];
void dfs(int v, int u = -1, int score = 0) {
  dist[v] = score;
  for (auto c : dat[v]) {
    if (c == u)
      continue;
    dfs(c, v, score + 1);
  }
}
vi f(int x) {
  dist = vi(n);
  dfs(x);
  return dist;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int u, v;
  IN(n, u, v);
  u--;
  v--;
  rep0(i, n - 1) {
    int a, b;
    IN(a, b);
    a--;
    b--;
    dat[a].pb(b);
    dat[b].pb(a);
  }
  vi distU = f(u);
  vi distV = f(v);

  int mx = 0;
  rep0(i, n) {
    if (distV[i] > distU[i]) {
      chmax(mx, distV[i]);
    }
  }
  mx--;
  OUT(mx);
  return 0;
}