Language: C++

v#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <random>
#include <iomanip>
#include <functional>
#include <cassert>

using namespace std;

typedef long long ll;

int main() {
  ios_base::sync_with_stdio(false); cin.tie(0);
#ifdef LOCAL
  freopen("input.txt", "r", stdin);
#endif
  int n, m;
  cin >> n >> m;
  vector <pair <int, int>> edges;
  vector <vector <int>> g(n);
  vector <int> deg(n);
  for (int i = 0; i < m; ++i) {
    int u, v;
    cin >> u >> v;
    --u, --v;
    edges.emplace_back(u, v);
    ++deg[u];
    g[u].push_back(i);
    g[v].push_back(i);
  } 
  if (m % 2 == 1) {
    cout << "-1\n";
    return 0;
  }
  vector <int> par(n), par_id(n), used(n), tin(n), tout(n);
  vector <pair <int, int>> who;
  int timer = 0;
  function <int(int, int, int)> dfs = [&] (int u, int p, int p_id) {
    used[u] = 1;
    par[u] = p;
    par_id[u] = p_id;
    tin[u] = timer++;
    vector <int> have;
    for (auto id : g[u]) {
      int v = edges[id].first ^ edges[id].second ^ u;
      if (!used[v]) {
        int ret = dfs(v, u, id);
        if (ret != -1) {
          have.push_back(ret);
        } 
      }
    }
    if (deg[u] % 2 == 1) {
      have.push_back(u);
    }
    for (int i = 1; i < (int)have.size(); i += 2) {
      who.emplace_back(have[i - 1], have[i]);
    }
    tout[u] = timer++;
    if ((int)have.size() % 2 == 1) return have.back();
    else return -1;
  };
  dfs(0, -1, -1);
  auto anc = [&] (int a, int b) {
    return tin[a] <= tin[b] && tout[b] <= tout[a];
  };
  for (auto p : who) {
    int a = p.first, b = p.second;
    int c = a;
    while (!anc(c, b)) {
      int i = par_id[c];
      swap(edges[i].first, edges[i].second);
      c = par[c];
    }
    c = b;
    while (!anc(c, a)) {
      int i = par_id[c];
      swap(edges[i].first, edges[i].second);
      c = par[c];
    }
  }
  for (auto p : edges) {
    cout << p.first + 1 << ' ' << p.second + 1 << '\n';
  }
}



Compiler output:
a.cc:1:2: error: stray '#' in program
    1 | v#include <iostream>
      |  ^
a.cc:1:1: error: 'v' does not name a type
    1 | v#include <iostream>
      | ^
In file included from /usr/include/c++/14/bits/char_traits.h:42,
                 from /usr/include/c++/14/string:42,
                 from a.cc:2:
/usr/include/c++/14/bits/postypes.h:68:11: error: 'ptrdiff_t' does not name a type
   68 |   typedef ptrdiff_t     streamsize; // Signed integral type
      |           ^~~~~~~~~
/usr/include/c++/14/bits/postypes.h:41:1: note: 'ptrdiff_t' is defined in header '<cstddef>'; this is probably fixable by adding '#include <cstddef>'
   40 | #include <cwchar> // For mbstate_t
  +++ |+#include <cstddef>
   41 | 
In file included from /usr/include/c++/14/bits/char_traits.h:50:
/usr/include/c++/14/type_traits:666:33: error: 'nullptr_t' is not a member of 'std'
  666 |     struct is_null_pointer<std::nullptr_t>
      |                                 ^~~~~~~~~
/usr/include/c++/14/type_traits:666:42: error: template argument 1 is invalid
  666 |     struct is_null_pointer<std::nullptr_t>
      |                                          ^
/usr/include/c++/14/type_traits:670:48: error: template argument 1 is invalid
  670 |     struct is_null_pointer<const std::nullptr_t>
      |                                                ^
/usr/include/c++/14/type_traits:674:51: error: template argument 1 is invalid
  674 |     struct is_null_pointer<volatile std::nullptr_t>
      |                                                   ^
/usr/include/c++/14/type_traits:678:57: error: template argument 1 is invalid
  678 |     struct is_null_pointer<const volatile std::nullptr_t>
      |                                                         ^
/usr/include/c++/14/type_traits:1429:37: error: 'size_t' is not a member of 'std'; did you mean 'size_t'?
 1429 |     : public integral_constant<std::size_t, alignof(_Tp)>
      |                                     ^~~~~~
In file included from /usr/include/wchar.h:35,
                 from /usr/include/c++/14/cwchar:44,
                 from /usr/include/c++/14/bits/postypes.h:40:
/usr/lib/gcc/x86_64-linux-gnu/14/include/stddef.h:214:23: note: 'size_t' declared here
  214 | typedef __SIZE_TYPE__ size_t;
      |                       ^~~~~~
/usr/include/c++/14/type_traits:1429:57: error: template argument 1 is invalid
 1429 |     : public integral_constant<std::size_t, alignof(_Tp)>
      |                                                         ^
/usr/include/c++/14/type_traits:1429:57: note: invalid template non-type parameter
/usr/include/c++/14/type_traits:1438:37: error: 'size_t' is not a member of 'std'; did you mean 'size_t'?
 1438 |     : public integral_constant<std::size_t, 0> { };
      |                                     ^~~~~~
/usr/lib/gcc/x86_64-linux-gnu/14/include/stddef.h:214:23: note: 'size_t' declared here
  214 | typedef __SIZE_TYPE__ size_t;
      |                       ^~~~~~
/usr/include/c++/14/type_traits:1438:46: error: template argument 1 is invalid
 1438 |     : public integral_constant<std::size_t, 0> { };
      |                                              ^
/usr/include/c++/14/type_traits:1438:46: note: invalid template non-type parameter
/usr/include/c++/14/type_traits:1440:26: error: 'std::size_t' has not been declared
 1440 |   template<typename _Tp, std::size_t _Size>
      |                          ^~~
/usr/include/c++/14/type_traits:1441:21: error: '_Size' was not declared in this scope
 1441 |     struct rank<_Tp[_Size]>
      |                     ^~~~~
/usr/include/c++/14/type_traits:1441:27: error: template argument 1 is invalid
 1441 |     struct rank<_Tp[_Size]>
      |                           ^
/usr/include/c++/14/type_traits:1442:37: error: 'size_t' is not a member of 'std'; did you mean 'size_t'?
 1442 |     : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };
      |                                     ^~~~~~
/usr/lib/gcc/x86_64-linux-gnu/14/include/stddef.h:214:23: note: 'size_t' declared here
  214 | typedef __SIZE_TYPE__ size_t;
      |                       ^~~~~~
/usr/include/c++/14/type_traits:1442:65: error: template argument 1 is invalid
 1442 |     : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };
      |                                                                 ^
/usr/include/c++/14/type_traits:1442:65: note: invalid template non-type parameter
/usr/include/c++/14/type_traits:1446:37: error: 'size_t' is not a member of 'std'; did you mean 'size_t'?
 1446 |     : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };
      |                                     ^~~~~~
/usr/lib/gcc/x86_64-linux-gnu/14/include/stddef.h:214:23: note: 'size_t' declared here
  214 | typedef __SIZE_TYPE__ size_t;
      |                       ^~~~~~
/usr/include/c++/14/type_traits:1446:65: error: template argument 1 is invalid
 1446 |     : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };
      |                                                                 ^
/usr/include/c++/14/type_traits:1446:65: note: invalid template non-type parameter
/usr/include/c++/14/type_traits:2086:26: error: 'std::size_t' has not been declared
 2086 |   template<typename _Tp, std::size_t _Size>
      |                          ^~~
/usr/include/c++/14/type_traits:2087:30: error: '_Size' was not declared in this scope
 2087 |     struct remove_extent<_Tp[_Size]>
      |                              ^~~~~
/usr/include/c++/14/type_traits:2087:36: error: template argument 1 is invalid
 2087 |     struct remove_extent<_Tp[_Size]>
      |                                    ^
/usr/include/c++/14/type_traits:2099:26: error: 'std::size_t' has not been declared
 2099 |   template<typename _Tp, std::size_t _Size>
      |                          ^~~
/usr/include/c++/14/type_traits:2100:35: error: '_Size' was not declared in this scope
 2100 |     struct remove_all_extents<_Tp[_Size]>
      |                                   ^~~~~
/usr/include/c++/14/type_traits:2100:41: error: template argument 1 is invalid
 2100 |     struct remove_all_extents<_Tp[_Size]>
      |                                         ^
/usr/include/c++/14/type_traits:2171:12: error: 'std::size_t' has not been declared
 2171 |   template<std::size_t _Len>
      |            ^~~
/usr/include/c++/14/type_traits:2176:30: error: '_Len' was not declared in this scope
 2176 |         unsigned char __data[_Len];
      |                              ^~~~
/usr/include/c++/14/type_traits:2194:12: error: 'std::size_t' has not been declared
 2194 |   template<std::size_t _Len, std::size_t _Align =
      |            ^~~
/usr/include/c++/14/type_traits:2194:30: error: 'std::size_t' has not been declared
 2194 |   template<std::size_t _Len, std::size_t _Align =
      |                              ^~~
/usr/include/c++/14/type_traits:2195:55: error: '_Len' was not declared in this scope
 2195 |            __alignof__(typename __aligned_storage_msa<_Len>::__type)>
      |                                                       ^~~~
/usr/include/c++/14/type_traits:2195:59: error: template argument 1 is invalid
 2195 |            __alignof__(typename __aligned_storage_msa<_Len>::__type)>
      |                                                           ^
/usr/include/c++/14/type_traits:2202:30: error: '_Len' was not declared in this scope
 2202 |         unsigned char __data[_Len];
      |                              ^~~~
/usr/include/c++/14/type_traits:2203:44: error: '_Align' was not declared in this scope
 2203 |         struct __attribute__((__aligned__((_Align)))) { } __align;
      |                                            ^~~~~~
/usr/include/c++/14/bits/char_traits.h:144:61: error: 'std::size_t' has not been declared
  144 |       compare(const char_type* __s1, const char_type* __s2, std::size_t __n);
      |                                                             ^~~
/usr/include/c++/14/bits/char_traits.h:146:40: error: 'size_t' in namespace 'std' does not name a type
  146 |       static _GLIBCXX14_CONSTEXPR std::size_t
      |                                        ^~~~~~
/usr/include/c++/14/bits/char_traits.h:150:34: error: 'std::size_t' has not been declared
  150 |       find(const char_type* __s, std::size_t __n, const char_type& __a);
      |                                  ^~~
/usr/include/c++/14/bits/char_traits.h:153:52: error: 'std::size_t' has not been declared
  153 |       move(char_type* __s1, const char_type* __s2, std::size_t __n);
      |                                                    ^~~
/usr/include/c++/14/bits/char_traits.h:156:52: error: 'std::size_t' has not been declared
  156 |       copy(char_type* __s1, const char_type* __s2, std::size_t __n);
      |                                                    ^~~
/usr/include/c++/14/bits/char_traits.h:159:30: error: 'std::size_t' has not been declared
  159 |       assign(char_type* __s, std::size_t __n, char_type __a);
      |                              ^~~
/usr/include/c++/14/bits/char_traits.h:187:59: error: 'std::size_t' has not been declared
  187 |     compare(const char_type* __s1, const char_type* __s2, std::size_t __n)
      |                                                           ^~~
/usr/include/c++/14/bits/char_traits.h: In static member function 'static constexpr int __gnu_cxx::char_traits<_CharT>::compare(const char_type*, const char_type*, int)':
/usr/include/c++/14/bits/char_traits.h:189:17: error: 'size_t' is not a member of 'std'; did you mean 'size_t'?
  189 |       for (std::size_t __i = 0; __i < __n; ++__i)
      |                 ^~~~~~
/usr/lib/gcc/x86_64-linux-gnu/14/include/stddef.h:214:23: note: 'size_t' declared here
  214 | typedef __SIZE_TYPE__ size_t;
      |                       ^~~~~~
/usr/include/c++/14/bits/char_traits.h:189:33: error: '__i' was not declared in this scope; did you mean '__n'?
  189 |       for (std::size_t __i = 0; __i < __n; ++__i)
      |                                 ^~~
      |                                 __n
/usr/include/c++/14/bits/char_traits.h: At global scope:
/usr/include/c++/14/bits/char_traits.h:198:31: error: 'size_t' in namespace 'std' does not name a