Problem p03641 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for (ll i = (a); i > (b); --i)
#define REP(i, n) for (ll i = 0; i < (n); ++i)
#define REPR(i, n) for (ll i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()
#define PERM(c)                                                                \
  sort(All(c));                                                                \
  for (bool cp = true; cp; cp = next_permutation(All(c)))
#define MKORDER(n)                                                             \
  vector<ll> od(n);                                                            \
  iota(All(od), 0LL);

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  ll __VA_ARGS__;                                                              \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

// generic lambdas
template <typename F>
#if defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
[[nodiscard]]
#elif defined(__GNUC__) &&                                                     \
    (__GNUC__ > 3 || __GNUC__ == 3 && __GNUC_MINOR__ >= 4)
__attribute__((warn_unused_result))
#endif // defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
static inline constexpr decltype(auto)
fix(F &&f) noexcept {
  return [f = std::forward<F>(f)](auto &&...args) {
    return f(f, std::forward<decltype(args)>(args)...);
  };
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
const double PI = atan(1.0) * 4.0;
const ll dw[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dh[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

// うし木 0-indexed
template <typename T, typename E> struct SegmentTree {
  typedef function<T(T, T)> F;
  typedef function<T(T, E)> G;
  int n;
  F f;
  G g;
  T e1;
  vector<T> dat;
  SegmentTree(){};
  SegmentTree(int n_, F f, G g, T e1, vector<T> v = vector<T>())
      : f(f), g(g), e1(e1) {
    init(n_);
    if (n_ == (int)v.size())
      build(n_, v);
  }
  void init(int n_) {
    n = 1;
    while (n < n_)
      n *= 2;
    dat.clear();
    dat.resize(2 * n - 1, e1);
  }
  void build(int n_, vector<T> v) {
    for (int i = 0; i < n_; i++)
      dat[i + n - 1] = v[i];
    for (int i = n - 2; i >= 0; i--)
      dat[i] = f(dat[i * 2 + 1], dat[i * 2 + 2]);
  }
  void update(int k, E a) {
    k += n - 1;
    dat[k] = g(dat[k], a);
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = f(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }
  inline T query(int a, int b) {
    T vl = e1, vr = e1;
    for (int l = a + n, r = b + n; l < r; l >>= 1, r >>= 1) {
      if (l & 1)
        vl = f(vl, dat[(l++) - 1]);
      if (r & 1)
        vr = f(dat[(--r) - 1], vr);
    }
    return f(vl, vr);
  }
};

VL x(2);
VL pa;
VVL G;

signed main() {
  LCIN(N);
  VL p(N);
  G.resize(N);
  pa.resize(N);
  VECCIN(p);
  PL e1 = {LINF, LINF};
  auto f = [](PL a, PL b) { return min(a, b); };
  auto g = [](PL a, PL b) { return b; };
  using seg = SegmentTree<PL, PL>;
  vector<seg> segs(2, seg(N + 1, f, g, e1, VPL(N + 1, e1)));
  REP(i, N) { segs[i % 2].update(i, {p[i], i}); }
  auto dfs = fix([&](auto f, int l, int r, int idx) -> void {
    if (l + 1 == r) {
      G[idx].push_back(p[l]);
      pa[p[l]] = p[r];
      return;
    }
    PL p = segs[l % 2].query(l, r);
    PL q = segs[(l % 2) ^ 1].query(p.se + 1, r + 1);
    ll lr = p.se;
    ll rl = q.se;

    G[idx].emplace_back(p.fi);
    pa[p.fi] = q.fi;

    if (lr != l)
      f(f, l, lr - 1, p.fi);
    if (lr + 1 != rl)
      f(f, lr + 1, rl - 1, p.fi);
    if (rl != r)
      f(f, rl + 1, r, p.fi);
  });
  dfs(0, N - 1, 0);
  VL ans;
  PQG<ll> pq;
  pq.push(G[0].front());
  while (pq.size()) {
    ll t = pq.top();
    pq.pop();

    ans.emplace_back(t);
    ans.emplace_back(pa[t]);

    FOREACH(to, G[t]) pq.push(to);
  }
  Printv(ans);
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for (ll i = (a); i > (b); --i)
#define REP(i, n) for (ll i = 0; i < (n); ++i)
#define REPR(i, n) for (ll i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()
#define PERM(c)                                                                \
  sort(All(c));                                                                \
  for (bool cp = true; cp; cp = next_permutation(All(c)))
#define MKORDER(n)                                                             \
  vector<ll> od(n);                                                            \
  iota(All(od), 0LL);

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  ll __VA_ARGS__;                                                              \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

// generic lambdas
template <typename F>
#if defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
[[nodiscard]]
#elif defined(__GNUC__) &&                                                     \
    (__GNUC__ > 3 || __GNUC__ == 3 && __GNUC_MINOR__ >= 4)
__attribute__((warn_unused_result))
#endif // defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
static inline constexpr decltype(auto)
fix(F &&f) noexcept {
  return [f = std::forward<F>(f)](auto &&...args) {
    return f(f, std::forward<decltype(args)>(args)...);
  };
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
const double PI = atan(1.0) * 4.0;
const ll dw[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dh[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

// うし木 0-indexed
template <typename T, typename E> struct SegmentTree {
  typedef function<T(T, T)> F;
  typedef function<T(T, E)> G;
  int n;
  F f;
  G g;
  T e1;
  vector<T> dat;
  SegmentTree(){};
  SegmentTree(int n_, F f, G g, T e1, vector<T> v = vector<T>())
      : f(f), g(g), e1(e1) {
    init(n_);
    if (n_ == (int)v.size())
      build(n_, v);
  }
  void init(int n_) {
    n = 1;
    while (n < n_)
      n *= 2;
    dat.clear();
    dat.resize(2 * n - 1, e1);
  }
  void build(int n_, vector<T> v) {
    for (int i = 0; i < n_; i++)
      dat[i + n - 1] = v[i];
    for (int i = n - 2; i >= 0; i--)
      dat[i] = f(dat[i * 2 + 1], dat[i * 2 + 2]);
  }
  void update(int k, E a) {
    k += n - 1;
    dat[k] = g(dat[k], a);
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = f(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }
  inline T query(int a, int b) {
    T vl = e1, vr = e1;
    for (int l = a + n, r = b + n; l < r; l >>= 1, r >>= 1) {
      if (l & 1)
        vl = f(vl, dat[(l++) - 1]);
      if (r & 1)
        vr = f(dat[(--r) - 1], vr);
    }
    return f(vl, vr);
  }
};

VL x(2);
VL pa;
VVL G;

signed main() {
  LCIN(N);
  VL p(N);
  G.resize(N + 1);
  pa.resize(N + 1);
  VECCIN(p);
  PL e1 = {LINF, LINF};
  auto f = [](PL a, PL b) { return min(a, b); };
  auto g = [](PL a, PL b) { return b; };
  using seg = SegmentTree<PL, PL>;
  vector<seg> segs(2, seg(N + 1, f, g, e1, VPL(N + 1, e1)));
  REP(i, N) { segs[i % 2].update(i, {p[i], i}); }
  auto dfs = fix([&](auto f, int l, int r, int idx) -> void {
    if (l + 1 == r) {
      G[idx].push_back(p[l]);
      pa[p[l]] = p[r];
      return;
    }
    PL p = segs[l % 2].query(l, r);
    PL q = segs[(l % 2) ^ 1].query(p.se + 1, r + 1);
    ll lr = p.se;
    ll rl = q.se;

    G[idx].emplace_back(p.fi);
    pa[p.fi] = q.fi;

    if (lr != l)
      f(f, l, lr - 1, p.fi);
    if (lr + 1 != rl)
      f(f, lr + 1, rl - 1, p.fi);
    if (rl != r)
      f(f, rl + 1, r, p.fi);
  });
  dfs(0, N - 1, 0);
  VL ans;
  PQG<ll> pq;
  pq.push(G[0].front());
  while (pq.size()) {
    ll t = pq.top();
    pq.pop();

    ans.emplace_back(t);
    ans.emplace_back(pa[t]);

    FOREACH(to, G[t]) pq.push(to);
  }
  Printv(ans);
}