Problem p02850 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using pint = pair<int, int>;
using ll = long long;
using ull = unsigned long long;
using vll = vector<long long>;
using pll = pair<ll, ll>;
#define FOR(i, begin, end)                                                     \
  for (ll i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (ll i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define VREP(s, ite) for (auto ite = s.begin(); ite != s.end(); ++ite)
#define FI first
#define SE second
#define ALL(v) v.begin(), v.end()
// #define endl "\n"
#define ciosup                                                                 \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false);
#define eb emplace_back
#define vint vector<int>
constexpr ll INF = 1e15 + 7LL;
constexpr ll MOD = 1e9 + 7LL;

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (int i = 0; i < v.size(); ++i) {
    is >> v[i];
  }
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < v.size() - 1; ++i) {
    os << v[i] << " ";
  }
  if (v.size() > 0) {
    os << v[v.size() - 1] << endl;
  }
  return os;
}

struct edge {
  int to, color;
};

ll n, colornum;

vector<map<int, int>> G(100008);

void dfs(int now, int from) {
  int prevcolor = -1;
  if (from != -1) {
    for (auto elem : G[from]) {
      if (elem.FI == now) {
        prevcolor = elem.SE;
      }
    }
  }
  ll nowcolor = 1;
  for (auto elem : G[now]) {
    if (elem.FI != from) {
      if (nowcolor == prevcolor) {
        nowcolor++;
      }
      G[now][elem.FI] = nowcolor;
      dfs(elem.FI, now);
      colornum = max(colornum, nowcolor);
      ++nowcolor;
    }
  }
}

int main() {
  cin >> n;
  colornum = -1;
  vint avec(n - 1), bvec(n - 1);
  REP(i, n - 1) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    edge e;
    e.to = b;
    e.color = 0;
    G[a][b] = 0;
    G[b][a] = 0;
    avec[i] = a;
    bvec[i] = b;
  }
  dfs(0, -1);
  cout << colornum << endl;
  REP(i, n - 1) {
    int a = avec[i], b = bvec[i];
    if (G[a][b] != 0) {
      cout << G[a][b] << endl;
    } else {
      cout << G[b][a] << endl;
    }
  }
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using pint = pair<int, int>;
using ll = long long;
using ull = unsigned long long;
using vll = vector<long long>;
using pll = pair<ll, ll>;
#define FOR(i, begin, end)                                                     \
  for (ll i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (ll i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define VREP(s, ite) for (auto ite = s.begin(); ite != s.end(); ++ite)
#define FI first
#define SE second
#define ALL(v) v.begin(), v.end()
// #define endl "\n"
#define ciosup                                                                 \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false);
#define eb emplace_back
#define vint vector<int>
constexpr ll INF = 1e15 + 7LL;
constexpr ll MOD = 1e9 + 7LL;

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (int i = 0; i < v.size(); ++i) {
    is >> v[i];
  }
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < v.size() - 1; ++i) {
    os << v[i] << " ";
  }
  if (v.size() > 0) {
    os << v[v.size() - 1] << endl;
  }
  return os;
}

struct edge {
  int to, color;
};

ll n, colornum;

vector<map<int, int>> G(100008);

void dfs(int now, int from) {
  int prevcolor = -1;
  if (from != -1) {
    prevcolor = G[from][now];
  }
  ll nowcolor = 1;
  for (auto elem : G[now]) {
    if (elem.FI != from) {
      if (nowcolor == prevcolor) {
        nowcolor++;
      }
      G[now][elem.FI] = nowcolor;
      dfs(elem.FI, now);
      colornum = max(colornum, nowcolor);
      ++nowcolor;
    }
  }
}

int main() {
  cin >> n;
  colornum = -1;
  vint avec(n - 1), bvec(n - 1);
  REP(i, n - 1) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    edge e;
    e.to = b;
    e.color = 0;
    G[a][b] = 0;
    G[b][a] = 0;
    avec[i] = a;
    bvec[i] = b;
  }
  dfs(0, -1);
  cout << colornum << endl;
  REP(i, n - 1) {
    int a = avec[i], b = bvec[i];
    if (G[a][b] != 0) {
      cout << G[a][b] << endl;
    } else {
      cout << G[b][a] << endl;
    }
  }
}