Problem p02976 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR                                                                   \
  if (DEBUG)                                                                   \
  cerr
#define MACRO(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, NAME, ...) NAME
#define pr(...)                                                                \
  CERR << MACRO(__VA_ARGS__, pr10, pr9, pr8, pr7, pr6, pr5, pr4, pr3, pr2,     \
                pr1)(__VA_ARGS__)                                              \
       << endl
#define pr1(a) (#a) << "=" << (a) << " "
#define pr2(a, b) pr1(a) << pr1(b)
#define pr3(a, b, c) pr1(a) << pr2(b, c)
#define pr4(a, b, c, d) pr1(a) << pr3(b, c, d)
#define pr5(a, b, c, d, e) pr1(a) << pr4(b, c, d, e)
#define pr6(a, b, c, d, e, f) pr1(a) << pr5(b, c, d, e, f)
#define pr7(a, b, c, d, e, f, g) pr1(a) << pr6(b, c, d, e, f, g)
#define pr8(a, b, c, d, e, f, g, h) pr1(a) << pr7(b, c, d, e, f, g, h)
#define pr9(a, b, c, d, e, f, g, h, i) pr1(a) << pr8(b, c, d, e, f, g, h, i)
#define pr10(a, b, c, d, e, f, g, h, i, j)                                     \
  pr1(a) << pr9(b, c, d, e, f, g, h, i, j)
#define prArr(a)                                                               \
  {                                                                            \
    CERR << (#a) << "={";                                                      \
    int i = 0;                                                                 \
    for (auto t : (a))                                                         \
      CERR << (i++ ? ", " : "") << t;                                          \
    CERR << "}" << endl;                                                       \
  }
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL << 60) + 1e9; // ~ 1.15 * 1e18
const Int mod = (1e9) + 7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int, Int>;
template <class T> T Max(T &a, T b) { return a = max(a, b); }
template <class T> T Min(T &a, T b) { return a = min(a, b); }
template <class T1, class T2> ostream &operator<<(ostream &o, pair<T1, T2> p) {
  return o << "(" << p.first << "," << p.second << ")";
}
template <class T1, class T2, class T3>
ostream &operator<<(ostream &o, tuple<T1, T2, T3> t) {
  return o << "(" << get<0>(t) << "," << get<1>(t) << "," << get<2>(t) << ")";
}
template <class T1, class T2> istream &operator>>(istream &i, pair<T1, T2> &p) {
  return i >> p.first >> p.second;
}
template <class T> ostream &operator<<(ostream &o, vector<T> a) {
  Int i = 0;
  for (T t : a)
    o << (i++ ? " " : "") << t;
  return o;
}
template <class T> istream &operator>>(istream &i, vector<T> &a) {
  for (T &t : a)
    i >> t;
  return i;
}
// INSERT ABOVE HERE

Int N;
vector<vector<Int>> G;
vector<set<Int>> T;

void makeTree(Int pos) {
  static Int used[100010];
  used[pos] = 1;
  for (Int to : G[pos]) {
    if (used[to])
      continue;
    used[to] = 1;
    makeTree(to);
    T[pos].insert(to);
  }
}

set<P> ans;
vector<Int> indeg, outdeg;

void dfs(Int pos, Int pre) {

  for (Int to : G[pos]) {
    if (to == pre)
      continue;
    if (T[pos].count(to))
      continue;
    if (ans.count(P(pos, to)))
      continue;
    if (ans.count(P(to, pos)))
      continue;
    ans.emplace(pos, to);
    outdeg[pos]++;
    indeg[to]++;
  }

  for (Int to : T[pos]) {
    if (to == pre)
      continue;
    dfs(to, pos);
  }

  if (pre == -1)
    return;
  if (indeg[pos] % 2 == 1) {
    ans.emplace(pre, pos);
    indeg[pos]++;
    outdeg[pre]++;
  } else {
    ans.emplace(pos, pre);
    outdeg[pos]++;
    indeg[pre]++;
  }
}

signed main() {
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int M;
  cin >> N >> M;
  G.resize(N);
  T.resize(N);
  indeg.resize(N);
  outdeg.resize(N);
  for (Int i = 0; i < M; i++) {
    Int a, b;
    cin >> a >> b;
    a--, b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  if (M % 2) {
    cout << -1 << endl;
    return 0;
  }

  makeTree(0);
  dfs(0, -1);

  for (auto p : ans)
    cout << p.first + 1 << " " << p.second + 1 << endl;

  vector<Int> in(N), out(N);
  for (auto p : ans)
    out[p.first]++, in[p.second]++;

  for (Int i = 0; i < N; i++) {
    assert(out[i] % 2 == 0);
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR                                                                   \
  if (DEBUG)                                                                   \
  cerr
#define MACRO(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, NAME, ...) NAME
#define pr(...)                                                                \
  CERR << MACRO(__VA_ARGS__, pr10, pr9, pr8, pr7, pr6, pr5, pr4, pr3, pr2,     \
                pr1)(__VA_ARGS__)                                              \
       << endl
#define pr1(a) (#a) << "=" << (a) << " "
#define pr2(a, b) pr1(a) << pr1(b)
#define pr3(a, b, c) pr1(a) << pr2(b, c)
#define pr4(a, b, c, d) pr1(a) << pr3(b, c, d)
#define pr5(a, b, c, d, e) pr1(a) << pr4(b, c, d, e)
#define pr6(a, b, c, d, e, f) pr1(a) << pr5(b, c, d, e, f)
#define pr7(a, b, c, d, e, f, g) pr1(a) << pr6(b, c, d, e, f, g)
#define pr8(a, b, c, d, e, f, g, h) pr1(a) << pr7(b, c, d, e, f, g, h)
#define pr9(a, b, c, d, e, f, g, h, i) pr1(a) << pr8(b, c, d, e, f, g, h, i)
#define pr10(a, b, c, d, e, f, g, h, i, j)                                     \
  pr1(a) << pr9(b, c, d, e, f, g, h, i, j)
#define prArr(a)                                                               \
  {                                                                            \
    CERR << (#a) << "={";                                                      \
    int i = 0;                                                                 \
    for (auto t : (a))                                                         \
      CERR << (i++ ? ", " : "") << t;                                          \
    CERR << "}" << endl;                                                       \
  }
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL << 60) + 1e9; // ~ 1.15 * 1e18
const Int mod = (1e9) + 7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int, Int>;
template <class T> T Max(T &a, T b) { return a = max(a, b); }
template <class T> T Min(T &a, T b) { return a = min(a, b); }
template <class T1, class T2> ostream &operator<<(ostream &o, pair<T1, T2> p) {
  return o << "(" << p.first << "," << p.second << ")";
}
template <class T1, class T2, class T3>
ostream &operator<<(ostream &o, tuple<T1, T2, T3> t) {
  return o << "(" << get<0>(t) << "," << get<1>(t) << "," << get<2>(t) << ")";
}
template <class T1, class T2> istream &operator>>(istream &i, pair<T1, T2> &p) {
  return i >> p.first >> p.second;
}
template <class T> ostream &operator<<(ostream &o, vector<T> a) {
  Int i = 0;
  for (T t : a)
    o << (i++ ? " " : "") << t;
  return o;
}
template <class T> istream &operator>>(istream &i, vector<T> &a) {
  for (T &t : a)
    i >> t;
  return i;
}
// INSERT ABOVE HERE

Int N;
vector<vector<Int>> G;
vector<set<Int>> T;

void makeTree(Int pos) {
  static Int used[100010];
  used[pos] = 1;
  for (Int to : G[pos]) {
    if (used[to])
      continue;
    used[to] = 1;
    makeTree(to);
    T[pos].insert(to);
  }
}

set<P> ans;
vector<Int> indeg, outdeg;

void dfs(Int pos, Int pre) {

  for (Int to : G[pos]) {
    if (to == pre)
      continue;
    if (T[pos].count(to))
      continue;
    if (ans.count(P(pos, to)))
      continue;
    if (ans.count(P(to, pos)))
      continue;
    ans.emplace(pos, to);
    outdeg[pos]++;
    indeg[to]++;
  }

  for (Int to : T[pos]) {
    if (to == pre)
      continue;
    dfs(to, pos);
  }

  if (pre == -1)
    return;
  if (outdeg[pos] % 2 == 0) {
    ans.emplace(pre, pos);
    indeg[pos]++;
    outdeg[pre]++;
  } else {
    ans.emplace(pos, pre);
    outdeg[pos]++;
    indeg[pre]++;
  }
}

signed main() {
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int M;
  cin >> N >> M;
  G.resize(N);
  T.resize(N);
  indeg.resize(N);
  outdeg.resize(N);
  for (Int i = 0; i < M; i++) {
    Int a, b;
    cin >> a >> b;
    a--, b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  if (M % 2) {
    cout << -1 << endl;
    return 0;
  }

  makeTree(0);
  dfs(0, -1);

  for (auto p : ans)
    cout << p.first + 1 << " " << p.second + 1 << endl;

  vector<Int> in(N), out(N);
  for (auto p : ans)
    out[p.first]++, in[p.second]++;

  for (Int i = 0; i < N; i++) {
    assert(out[i] % 2 == 0);
  }

  return 0;
}