Problem p03647 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n)-1; i >= 0; i--)
#define inf 0x3f3f3f3f3f3f3f3f
#define ALL(a) (a).begin(), (a).end()
#define DEBUG(x) cout << #x << ": " << x << endl
#define ll long long
#define ull unsigned long long
using pii = pair<ll, ll>;
#define eps 1e-14
#define SETUP                                                                  \
  cin.tie(0), ios::sync_with_stdio(false),                                     \
      cout << setprecision(15) << std::fixed;

template <class T> using vec2 = std::vector<vector<T>>;

namespace {
struct input_returnner {
  ll N;
  input_returnner(ll N_ = 0) : N(N_) {}
  template <typename T> operator vector<T>() const {
    vector<T> res(N);
    for (auto &a : res)
      cin >> a;
    return std::move(res);
  }
  template <typename T> operator T() const {
    T res;
    cin >> res;
    return res;
  }
  template <typename T> T operator-(T right) {
    return T(input_returnner()) - right;
  }
  template <typename T> T operator+(T right) {
    return T(input_returnner()) + right;
  }
  template <typename T> T operator*(T right) {
    return T(input_returnner()) * right;
  }
  template <typename T> T operator/(T right) {
    return T(input_returnner()) / right;
  }
  template <typename T> T operator<<(T right) {
    return T(input_returnner()) << right;
  }
  template <typename T> T operator>>(T right) {
    return T(input_returnner()) >> right;
  }
};
template <typename T> input_returnner in() { return in<T>(); }
input_returnner in() { return input_returnner(); }
input_returnner in(ll N) { return std::move(input_returnner(N)); }
} // namespace

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

template <typename T> struct is_vector : std::false_type {};

template <typename T> struct is_vector<std::vector<T>> : std::true_type {};

template <typename T> constexpr bool is_vector_v = is_vector<T>::value;

template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  if (!v.empty()) {
    for (int i = 0; i < v.size(); ++i) {
      out << v[i] << (i == v.size() - 1 ? "\n" : (is_vector_v<T> ? "" : ", "));
    }
  }
  return out;
}

namespace std {
// ref:
// https://stackoverflow.com/questions/7110301/generic-hash-for-tuples-in-unordered-map-unordered-set
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
  seed ^= std::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
  static void apply(size_t &seed, Tuple const &tuple) {
    HashValueImpl<Tuple, Index - 1>::apply(seed, tuple);
    hash_combine(seed, std::get<Index>(tuple));
  }
};

template <class Tuple> struct HashValueImpl<Tuple, 0> {
  static void apply(size_t &seed, Tuple const &tuple) {
    hash_combine(seed, std::get<0>(tuple));
  }
};
template <typename... TT> struct hash<std::tuple<TT...>> {
  size_t operator()(std::tuple<TT...> const &tt) const {
    size_t seed = 0;
    HashValueImpl<std::tuple<TT...>>::apply(seed, tt);
    return seed;
  }
};

template <class T, class U> class hash<std::pair<T, U>> {
public:
  size_t operator()(const std::pair<T, U> &x) const {
    return hash<std::tuple<T, U>>()(std::tie(x.first, x.second));
  }
};
} // namespace std

// ref: https://stackoverflow.com/questions/6245735/pretty-print-stdtuple
namespace aux {
template <std::size_t...> struct seq {};

template <std::size_t N, std::size_t... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> {};

template <std::size_t... Is> struct gen_seq<0, Is...> : seq<Is...> {};

template <class Ch, class Tr, class Tuple, std::size_t... Is>
void print_tuple(std::basic_ostream<Ch, Tr> &os, Tuple const &t, seq<Is...>) {
  using swallow = int[];
  (void)swallow{0,
                (void(os << (Is == 0 ? "" : ", ") << std::get<Is>(t)), 0)...};
}
} // namespace aux

template <class Ch, class Tr, class... Args>
auto operator<<(std::basic_ostream<Ch, Tr> &os, std::tuple<Args...> const &t)
    -> std::basic_ostream<Ch, Tr> & {
  os << "(";
  aux::print_tuple(os, t, aux::gen_seq<sizeof...(Args)>());
  return os << ")";
}

template <class S, class T>
std::ostream &operator<<(std::ostream &os, const std::pair<S, T> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}

// ref:
// https://stackoverflow.com/questions/8542591/c11-reverse-range-based-for-loo�Fp
template <typename T> struct reversion_wrapper {
  T &iterable;
};

template <typename T> auto begin(reversion_wrapper<T> w) {
  return std::rbegin(w.iterable);
}

template <typename T> auto end(reversion_wrapper<T> w) {
  return std::rend(w.iterable);
}

template <typename T> reversion_wrapper<T> REV(T &&iterable) {
  return {iterable};
}

ll MOD = 1e9 + 7;

void solve();

signed main() {
  SETUP;
  solve();
#ifdef _DEBUG
  system("pause");
#endif
  return 0;
}

// template

#define int long long

void solve() {
  int N, M;
  cin >> N >> M;
  vector<vector<int>> G(N, vector<int>(N));
  unordered_set<int> zero, enu;
  REP(i, M) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    if (a == 0)
      zero.insert(b);
    if (b == N - 1)
      enu.insert(a);
  }

  for (auto &a : zero) {
    if (enu.count(a) == 1) {
      cout << "POSSIBLE" << endl;
      return;
    }
  }
  cout << "IMPOSSIBLE" << endl;
}


Fixed submission:
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n)-1; i >= 0; i--)
#define inf 0x3f3f3f3f3f3f3f3f
#define ALL(a) (a).begin(), (a).end()
#define DEBUG(x) cout << #x << ": " << x << endl
#define ll long long
#define ull unsigned long long
using pii = pair<ll, ll>;
#define eps 1e-14
#define SETUP                                                                  \
  cin.tie(0), ios::sync_with_stdio(false),                                     \
      cout << setprecision(15) << std::fixed;

template <class T> using vec2 = std::vector<vector<T>>;

namespace {
struct input_returnner {
  ll N;
  input_returnner(ll N_ = 0) : N(N_) {}
  template <typename T> operator vector<T>() const {
    vector<T> res(N);
    for (auto &a : res)
      cin >> a;
    return std::move(res);
  }
  template <typename T> operator T() const {
    T res;
    cin >> res;
    return res;
  }
  template <typename T> T operator-(T right) {
    return T(input_returnner()) - right;
  }
  template <typename T> T operator+(T right) {
    return T(input_returnner()) + right;
  }
  template <typename T> T operator*(T right) {
    return T(input_returnner()) * right;
  }
  template <typename T> T operator/(T right) {
    return T(input_returnner()) / right;
  }
  template <typename T> T operator<<(T right) {
    return T(input_returnner()) << right;
  }
  template <typename T> T operator>>(T right) {
    return T(input_returnner()) >> right;
  }
};
template <typename T> input_returnner in() { return in<T>(); }
input_returnner in() { return input_returnner(); }
input_returnner in(ll N) { return std::move(input_returnner(N)); }
} // namespace

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

template <typename T> struct is_vector : std::false_type {};

template <typename T> struct is_vector<std::vector<T>> : std::true_type {};

template <typename T> constexpr bool is_vector_v = is_vector<T>::value;

template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  if (!v.empty()) {
    for (int i = 0; i < v.size(); ++i) {
      out << v[i] << (i == v.size() - 1 ? "\n" : (is_vector_v<T> ? "" : ", "));
    }
  }
  return out;
}

namespace std {
// ref:
// https://stackoverflow.com/questions/7110301/generic-hash-for-tuples-in-unordered-map-unordered-set
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
  seed ^= std::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
  static void apply(size_t &seed, Tuple const &tuple) {
    HashValueImpl<Tuple, Index - 1>::apply(seed, tuple);
    hash_combine(seed, std::get<Index>(tuple));
  }
};

template <class Tuple> struct HashValueImpl<Tuple, 0> {
  static void apply(size_t &seed, Tuple const &tuple) {
    hash_combine(seed, std::get<0>(tuple));
  }
};
template <typename... TT> struct hash<std::tuple<TT...>> {
  size_t operator()(std::tuple<TT...> const &tt) const {
    size_t seed = 0;
    HashValueImpl<std::tuple<TT...>>::apply(seed, tt);
    return seed;
  }
};

template <class T, class U> class hash<std::pair<T, U>> {
public:
  size_t operator()(const std::pair<T, U> &x) const {
    return hash<std::tuple<T, U>>()(std::tie(x.first, x.second));
  }
};
} // namespace std

// ref: https://stackoverflow.com/questions/6245735/pretty-print-stdtuple
namespace aux {
template <std::size_t...> struct seq {};

template <std::size_t N, std::size_t... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> {};

template <std::size_t... Is> struct gen_seq<0, Is...> : seq<Is...> {};

template <class Ch, class Tr, class Tuple, std::size_t... Is>
void print_tuple(std::basic_ostream<Ch, Tr> &os, Tuple const &t, seq<Is...>) {
  using swallow = int[];
  (void)swallow{0,
                (void(os << (Is == 0 ? "" : ", ") << std::get<Is>(t)), 0)...};
}
} // namespace aux

template <class Ch, class Tr, class... Args>
auto operator<<(std::basic_ostream<Ch, Tr> &os, std::tuple<Args...> const &t)
    -> std::basic_ostream<Ch, Tr> & {
  os << "(";
  aux::print_tuple(os, t, aux::gen_seq<sizeof...(Args)>());
  return os << ")";
}

template <class S, class T>
std::ostream &operator<<(std::ostream &os, const std::pair<S, T> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}

// ref:
// https://stackoverflow.com/questions/8542591/c11-reverse-range-based-for-loo�Fp
template <typename T> struct reversion_wrapper {
  T &iterable;
};

template <typename T> auto begin(reversion_wrapper<T> w) {
  return std::rbegin(w.iterable);
}

template <typename T> auto end(reversion_wrapper<T> w) {
  return std::rend(w.iterable);
}

template <typename T> reversion_wrapper<T> REV(T &&iterable) {
  return {iterable};
}

ll MOD = 1e9 + 7;

void solve();

signed main() {
  SETUP;
  solve();
#ifdef _DEBUG
  system("pause");
#endif
  return 0;
}

// template

#define int long long

void solve() {
  int N, M;
  cin >> N >> M;
  unordered_set<int> zero, enu;
  REP(i, M) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    if (a == 0)
      zero.insert(b);
    if (b == N - 1)
      enu.insert(a);
  }

  for (auto &a : zero) {
    if (enu.count(a) == 1) {
      cout << "POSSIBLE" << endl;
      return;
    }
  }
  cout << "IMPOSSIBLE" << endl;
}