Problem p02976 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdint>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
using ll = long long;
using ull = unsigned long long;

constexpr ll INF = 1'000'000'000'000'000'000;
namespace utils {
#define ALL(x) begin(x), end(x)
#define rALL(x) rbegin(x), rend(x)
template <class T, class Compare>
using p_queue = priority_queue<T, vector<T>, Compare>;
template <class T> using min_queue = p_queue<T, greater<T>>;
template <class T> using max_queue = p_queue<T, less<T>>;

template <class T> bool min_update(T &X, const T &A) {
  if (X > A) {
    X = A;
    return true;
  }
  return false;
}
template <class T> bool max_update(T &X, const T &A) {
  if (X < A) {
    X = A;
    return true;
  }
  return false;
}
using V_Set = unordered_set<int>;
using E_Set = unordered_map<int, V_Set>;

ll operator"" _64(unsigned long long x) { return ll(x); }
ull operator"" _64u(unsigned long long x) { return ull(x); }

template <class T> vector<T> make_vector(int n, T t) { return vector<T>(n, t); }
template <class... Ts> auto make_vector(int n, Ts... ts) {
  return vector<decltype(make_vector(ts...))>(n, make_vector(ts...));
}

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &&x : v) {
    os << x << ' ';
  }
  cout << endl;
  return os;
}
} // namespace utils
using namespace utils;

class solver {
  istream &is;
  ostream &os;

  E_Set outE, E;
  ll N, M;
  vector<bool> vis;
  vector<pair<int, int>> sE;

public:
  solver(istream &I, ostream &O) : is(I), os(O) {}

  void make_spanning(int v) {
    vis[v] = true;
    for (auto &&u : E[v]) {
      if (vis[u]) {
        continue;
      }
      make_spanning(u);
      sE.emplace_back(v, u);
    } // end u
  }

  void run() {
    is >> N >> M;
    if (!is)
      return;
    for (int i = 0; i < M; ++i) {
      int A, B;
      is >> A >> B;
      E[A].insert(B);
      E[B].insert(A);
    } // end i
    vis.resize(N, false);
    if (M % 2) {
      cout << -1 << endl;
      return;
    }
    make_spanning(1);
    assert(sE.size() == N - 1);

    for (auto &&p = sE.begin(); p != sE.end(); p++) {
      int v, u;
      tie(v, u) = *p;
      for (auto &&t : E[u]) {
        if (t == v)
          continue;
        outE[t].insert(u);
        E[t].erase(u);
      } // end t
      E[v].erase(u);
      E.erase(u);
      if (outE[u].size() % 2) {
        outE[u].insert(v);
      } else {
        outE[v].insert(u);
      }
    }
    for (auto &&et : outE) {
      int u = et.first;
      for (auto &&v : et.second) {
        cout << u << ' ' << v << endl;
      } // end v
    }   // end et
  }
};

int main(int argc, char *argv[]) {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << setprecision(16) << scientific;
#ifdef DEBUG
  string test_cases = "test_EvenDegrees.txt";
  cerr << test_cases << " -->" << endl;
  auto fs = fstream(test_cases, fstream::in);
  int loop = 0;
  while (fs) {
    loop++;
    cout << '#' << loop << "#------\n";
    solver(fs, cout).run();
  }
  if (loop <= 1) {
    cout << "===" << endl;
    while (cin)
      solver(cin, cout).run();
  }
#else
  solver(cin, cout).run();
#endif
  return 0;
}


Fixed submission:
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdint>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
using ll = long long;
using ull = unsigned long long;

constexpr ll INF = 1'000'000'000'000'000'000;
namespace utils {
#define ALL(x) begin(x), end(x)
#define rALL(x) rbegin(x), rend(x)
template <class T, class Compare>
using p_queue = priority_queue<T, vector<T>, Compare>;
template <class T> using min_queue = p_queue<T, greater<T>>;
template <class T> using max_queue = p_queue<T, less<T>>;

template <class T> bool min_update(T &X, const T &A) {
  if (X > A) {
    X = A;
    return true;
  }
  return false;
}
template <class T> bool max_update(T &X, const T &A) {
  if (X < A) {
    X = A;
    return true;
  }
  return false;
}
using V_Set = unordered_set<int>;
using E_Set = unordered_map<int, V_Set>;

ll operator"" _64(unsigned long long x) { return ll(x); }
ull operator"" _64u(unsigned long long x) { return ull(x); }

template <class T> vector<T> make_vector(int n, T t) { return vector<T>(n, t); }
template <class... Ts> auto make_vector(int n, Ts... ts) {
  return vector<decltype(make_vector(ts...))>(n, make_vector(ts...));
}

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &&x : v) {
    os << x << ' ';
  }
  cout << endl;
  return os;
}
} // namespace utils
using namespace utils;

class solver {
  istream &is;
  ostream &os;

  E_Set outE, E;
  ll N, M;
  vector<bool> vis;
  vector<pair<int, int>> sE;

public:
  solver(istream &I, ostream &O) : is(I), os(O) {}

  void make_spanning(int v) {
    vis[v] = true;
    for (auto &&u : E[v]) {
      if (vis[u]) {
        continue;
      }
      make_spanning(u);
      sE.emplace_back(v, u);
    } // end u
  }

  void run() {
    is >> N >> M;
    if (!is)
      return;
    for (int i = 0; i < M; ++i) {
      int A, B;
      is >> A >> B;
      E[A].insert(B);
      E[B].insert(A);
    } // end i
    vis.resize(N + 1, false);
    if (M % 2) {
      cout << -1 << endl;
      return;
    }
    make_spanning(1);
    assert(sE.size() == N - 1);

    for (auto &&p = sE.begin(); p != sE.end(); p++) {
      int v, u;
      tie(v, u) = *p;
      for (auto &&t : E[u]) {
        if (t == v)
          continue;
        outE[t].insert(u);
        E[t].erase(u);
      } // end t
      E[v].erase(u);
      E.erase(u);
      if (outE[u].size() % 2) {
        outE[u].insert(v);
      } else {
        outE[v].insert(u);
      }
    }
    for (auto &&et : outE) {
      int u = et.first;
      for (auto &&v : et.second) {
        cout << u << ' ' << v << endl;
      } // end v
    }   // end et
  }
};

int main(int argc, char *argv[]) {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << setprecision(16) << scientific;
#ifdef DEBUG
  string test_cases = "test_EvenDegrees.txt";
  cerr << test_cases << " -->" << endl;
  auto fs = fstream(test_cases, fstream::in);
  int loop = 0;
  while (fs) {
    loop++;
    cout << '#' << loop << "#------\n";
    solver(fs, cout).run();
  }
  if (loop <= 1) {
    cout << "===" << endl;
    while (cin)
      solver(cin, cout).run();
  }
#else
  solver(cin, cout).run();
#endif
  return 0;
}