Problem p02976 (C++)

Buggy submission (status: Runtime Error):
#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

using namespace std;
typedef unsigned int uint;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<LL, LL> PP;
#define REP(i, a, n) for (LL i = (a), i##_max = (n); i < i##_max; ++i)
#define REM(i, a, n) for (LL i = (LL)(n)-1, i##min = (a); i >= i##min; --i)
#define ALL(arr) (arr).begin(), (arr).end()
#define FLOAT fixed << setprecision(16)
#define SPEEDUP                                                                \
  {                                                                            \
    cin.tie(NULL);                                                             \
    ios::sync_with_stdio(false);                                               \
  }
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream &operator>>(istream &ist, pair<T, U> &right) {
  return ist >> right.first >> right.second;
}
template <class T, class U>
ostream &operator<<(ostream &ost, const pair<T, U> &right) {
  return ost << right.first << ' ' << right.second;
}
template <class T, class TCompatible, size_t N>
void Fill(T (&dest)[N], const TCompatible &val) {
  fill(dest, dest + N, val);
}
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T (&dest)[M][N], const TCompatible &val) {
  for (int i = 0; i < M; ++i)
    Fill(dest[i], val);
}
template <class T> T Compare(T left, T right) {
  return left > right ? 1 : (left < right ? -1 : 0);
}
istream &Ignore(istream &ist) {
  string s;
  ist >> s;
  return ist;
}
bool Inside(int i, int j, int h, int w) {
  return i >= 0 && i < h && j >= 0 && j < w;
}
template <class T> T Next() {
  T buf;
  cin >> buf;
  return buf;
}

#ifdef ONLY_MY_ENVIR
#include "BIT.h"
#include "BinaryMatrix.h"
#include "Factorization.h"
#include "FlowSolver.h"
#include "Graph.h"
#include "IntMod.h"
#include "LazySegmentTree.h"
#include "Math.h"
#include "Matrix.h"
#include "MinMax.h"
#include "Position.h"
#include "Range.h"
#include "Rational.h"
#include "SegmentTree.h"
#include "SegmentTree2D.h"
#include "SuffixArray.h"
#include "Tree.h"
#include "UnionFind.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream &operator>>(istream &ist, __int128 &val) {
  LL tmp;
  ist >> tmp;
  val = tmp;
  return ist;
}
ostream &operator<<(ostream &ost, __int128 val) {
  LL tmp = val;
  ost << tmp;
  return ost;
}
#endif

#if 1234567891
#include <array>
#include <random>
#include <unordered_map>
#include <unordered_set>
template <typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T>>;
// template <class T>
// auto Is(const T& value) { return [value](const auto& comparand) -> bool {
// return comparand == value; }; }
#endif

int N, M;
vector<int> G[100000];
int used[10000];

void print(int from, int to) { cout << from + 1 << ' ' << to + 1 << endl; }

bool DFS(int from, int prev) {
  if (used[from] == 1)
    return false;
  if (used[from] == 2)
    return true;
  used[from] = 1;

  vector<int> edges;
  for (int to : G[from]) {
    if (to == prev)
      continue;
    if (!DFS(to, from)) {
      edges.push_back(to);
    }
  }

  int K = edges.size() / 2;
  REP(i, 0, K) {
    print(from, edges[2 * i]);
    print(from, edges[2 * i + 1]);
  }

  used[from] = 2;
  if (edges.size() % 2 == 0) {
    return false;
  } else {
    print(from, edges.back());
    print(from, prev);
    return true;
  }
}

int main() {
  cin >> N >> M;
  REP(i, 0, M) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  if (M % 2 == 0) {
    DFS(0, -1);
  } else {
    cout << -1 << endl;
  }
  return 0;
}

Fixed submission:
#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

using namespace std;
typedef unsigned int uint;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<LL, LL> PP;
#define REP(i, a, n) for (LL i = (a), i##_max = (n); i < i##_max; ++i)
#define REM(i, a, n) for (LL i = (LL)(n)-1, i##min = (a); i >= i##min; --i)
#define ALL(arr) (arr).begin(), (arr).end()
#define FLOAT fixed << setprecision(16)
#define SPEEDUP                                                                \
  {                                                                            \
    cin.tie(NULL);                                                             \
    ios::sync_with_stdio(false);                                               \
  }
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream &operator>>(istream &ist, pair<T, U> &right) {
  return ist >> right.first >> right.second;
}
template <class T, class U>
ostream &operator<<(ostream &ost, const pair<T, U> &right) {
  return ost << right.first << ' ' << right.second;
}
template <class T, class TCompatible, size_t N>
void Fill(T (&dest)[N], const TCompatible &val) {
  fill(dest, dest + N, val);
}
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T (&dest)[M][N], const TCompatible &val) {
  for (int i = 0; i < M; ++i)
    Fill(dest[i], val);
}
template <class T> T Compare(T left, T right) {
  return left > right ? 1 : (left < right ? -1 : 0);
}
istream &Ignore(istream &ist) {
  string s;
  ist >> s;
  return ist;
}
bool Inside(int i, int j, int h, int w) {
  return i >= 0 && i < h && j >= 0 && j < w;
}
template <class T> T Next() {
  T buf;
  cin >> buf;
  return buf;
}

#ifdef ONLY_MY_ENVIR
#include "BIT.h"
#include "BinaryMatrix.h"
#include "Factorization.h"
#include "FlowSolver.h"
#include "Graph.h"
#include "IntMod.h"
#include "LazySegmentTree.h"
#include "Math.h"
#include "Matrix.h"
#include "MinMax.h"
#include "Position.h"
#include "Range.h"
#include "Rational.h"
#include "SegmentTree.h"
#include "SegmentTree2D.h"
#include "SuffixArray.h"
#include "Tree.h"
#include "UnionFind.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream &operator>>(istream &ist, __int128 &val) {
  LL tmp;
  ist >> tmp;
  val = tmp;
  return ist;
}
ostream &operator<<(ostream &ost, __int128 val) {
  LL tmp = val;
  ost << tmp;
  return ost;
}
#endif

#if 1234567891
#include <array>
#include <random>
#include <unordered_map>
#include <unordered_set>
template <typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T>>;
// template <class T>
// auto Is(const T& value) { return [value](const auto& comparand) -> bool {
// return comparand == value; }; }
#endif

int N, M;
vector<int> G[100000];
int used[100000];

void print(int from, int to) { cout << from + 1 << ' ' << to + 1 << endl; }

bool DFS(int from, int prev) {
  if (used[from] == 1)
    return false;
  if (used[from] == 2)
    return true;
  used[from] = 1;

  vector<int> edges;
  for (int to : G[from]) {
    if (to == prev)
      continue;
    if (!DFS(to, from)) {
      edges.push_back(to);
    }
  }

  int K = edges.size() / 2;
  REP(i, 0, K) {
    print(from, edges[2 * i]);
    print(from, edges[2 * i + 1]);
  }

  used[from] = 2;
  if (edges.size() % 2 == 0) {
    return false;
  } else {
    print(from, edges.back());
    print(from, prev);
    return true;
  }
}

int main() {
  cin >> N >> M;
  REP(i, 0, M) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  if (M % 2 == 0) {
    DFS(0, -1);
  } else {
    cout << -1 << endl;
  }
  return 0;
}