Problem p02749 (C++)

Buggy submission (status: Runtime Error):
//@formatter:off
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = int(s); i < int(n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vvd vector<vector<double>>
#define vs vector<string>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define vp vector<P>
#define vvp vector<vector<P>>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == int(v.size() - 1) ? "" : ",");
  return os << ']';
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << '\n'; }
void YES(bool b) { cout << (b ? "YES" : "NO") << '\n'; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n;
  cin >> n;
  vvi G(n);
  rep(_, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].pb(b);
    G[b].pb(a);
  }
  vvi ls(2);
  auto dfs = [&](auto &self, int v = 0, int p = -1, int c = 0) -> void {
    ls[c].pb(v);
    for (int u : G[v]) {
      if (u == p)
        continue;
      self(self, u, v, 1 - c);
    }
  };
  dfs(dfs);
  if (ls[0].size() < ls[1].size())
    swap(ls[0], ls[1]);
  int a = n / 3, b = n / 3;
  if (n % 3 == 2)
    a++, b++;
  else if (n % 3 == 1)
    a++;
  vi ans(n);
  if (ls[1].size() >= b) {
    rep(i, a) ans[ls[0][i]] = i * 3 + 1;
    rep(i, b) ans[ls[1][i]] = i * 3 + 2;
    int cnt = 0;
    rep2(i, a, ls[0].size()) {
      ans[ls[0][i]] = cnt * 3 + 3;
      cnt++;
    }
    rep2(i, b, ls[1].size()) {
      ans[ls[1][i]] = cnt * 3 + 3;
      cnt++;
    }
  } else {
    rep(i, a) ans[ls[0][i]] = i * 3 + 1;
    rep2(i, a, a + b) ans[ls[0][i]] = i * 3 + 2;
    int cnt = 0;
    rep2(i, a + b, ls[0].size()) {
      ans[ls[0][i]] = cnt * 3 + 3;
      cnt++;
    }
    rep(i, ls[1].size()) {
      ans[ls[1][i]] = cnt * 3 + 3;
      cnt++;
    }
  }
  for (int i : ans) {
    cout << i << ' ';
    assert(i <= n);
  }
  cout << endl;
}


Fixed submission:
//@formatter:off
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = int(s); i < int(n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vvd vector<vector<double>>
#define vs vector<string>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define vp vector<P>
#define vvp vector<vector<P>>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == int(v.size() - 1) ? "" : ",");
  return os << ']';
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << '\n'; }
void YES(bool b) { cout << (b ? "YES" : "NO") << '\n'; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n;
  cin >> n;
  vvi G(n);
  rep(_, n - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].pb(b);
    G[b].pb(a);
  }
  vvi ls(2);
  auto dfs = [&](auto &self, int v = 0, int p = -1, int c = 0) -> void {
    ls[c].pb(v);
    for (int u : G[v]) {
      if (u == p)
        continue;
      self(self, u, v, 1 - c);
    }
  };
  dfs(dfs);
  if (ls[0].size() < ls[1].size())
    swap(ls[0], ls[1]);
  int a = n / 3, b = n / 3;
  if (n % 3 == 2)
    a++, b++;
  else if (n % 3 == 1)
    a++;
  vi ans(n);
  if (ls[1].size() >= b) {
    rep(i, a) ans[ls[0][i]] = i * 3 + 1;
    rep(i, b) ans[ls[1][i]] = i * 3 + 2;
    int cnt = 0;
    rep2(i, a, ls[0].size()) {
      ans[ls[0][i]] = cnt * 3 + 3;
      cnt++;
    }
    rep2(i, b, ls[1].size()) {
      ans[ls[1][i]] = cnt * 3 + 3;
      cnt++;
    }
  } else {
    rep(i, a) ans[ls[0][i]] = i * 3 + 1;
    rep2(i, a, a + b) ans[ls[0][i]] = (i - a) * 3 + 2;
    int cnt = 0;
    rep2(i, a + b, ls[0].size()) {
      ans[ls[0][i]] = cnt * 3 + 3;
      cnt++;
    }
    rep(i, ls[1].size()) {
      ans[ls[1][i]] = cnt * 3 + 3;
      cnt++;
    }
  }
  for (int i : ans) {
    cout << i << ' ';
    assert(i <= n);
  }
  cout << endl;
}