Problem p03313 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x...)                                                              \
  do {                                                                         \
    cout << "\033[32;1m" << #x << "->";                                        \
    err(x);                                                                    \
  } while (0)
void err() { cout << "\033[39;0m" << endl; }
template <template <typename...> class T, typename t, typename... A>
void err(T<t> a, A... x) {
  for (auto v : a)
    cout << v << ' ';
  err(x...);
}
template <typename T, typename... A> void err(T a, A... x) {
  cout << a << ' ';
  err(x...);
}
#else
#define dbg(...)
#endif
typedef long long ll;
typedef pair<int, int> pi;
typedef vector<int> vi;
template <class T> using vc = vector<T>;
template <class T> using vvc = vc<vc<T>>;
template <class T> void mkuni(vector<T> &v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}
template <class T> void print(T x, int suc = 1) {
  cout << x;
  if (suc == 1)
    cout << '\n';
  else
    cout << ' ';
}
template <class T> void print(const vector<T> &v, int suc = 1) {
  for (int i = 0; i < v.size(); i++)
    print(v[i], i == (int)(v.size()) - 1 ? suc : 2);
}
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
const int maxn = 2e5 + 7;
#define pii pair<int, int>

pii a[maxn];
pii merge(pii a, pii b) {
  pii c;
  if (a.first < b.first)
    swap(a, b);
  c.first = a.first;
  c.second = max(a.second, b.first);
  return c;
}
int main() {
  int n;
  cin >> n;
  int pow2 = (1 << n);
  for (int i = 0, x; i < pow2; ++i) {
    cin >> x;
    a[i] = {x, 0};
  }
  for (int j = 0; j < n; ++j) {
    for (int i = 0; i < pow2; ++i) {
      if ((i >> j) & 1)
        a[i] = merge(a[i], a[i ^ (1 << j)]);
    }
  }
  int ans = 0;
  for (int i = 1; i < pow2; ++i) {
    print(max(ans, a[i].first + a[i].second));
    ans = max(ans, a[i].first + a[i].second);
  }
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x...)                                                              \
  do {                                                                         \
    cout << "\033[32;1m" << #x << "->";                                        \
    err(x);                                                                    \
  } while (0)
void err() { cout << "\033[39;0m" << endl; }
template <template <typename...> class T, typename t, typename... A>
void err(T<t> a, A... x) {
  for (auto v : a)
    cout << v << ' ';
  err(x...);
}
template <typename T, typename... A> void err(T a, A... x) {
  cout << a << ' ';
  err(x...);
}
#else
#define dbg(...)
#endif
typedef long long ll;
typedef pair<int, int> pi;
typedef vector<int> vi;
template <class T> using vc = vector<T>;
template <class T> using vvc = vc<vc<T>>;
template <class T> void mkuni(vector<T> &v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}
template <class T> void print(T x, int suc = 1) {
  cout << x;
  if (suc == 1)
    cout << '\n';
  else
    cout << ' ';
}
template <class T> void print(const vector<T> &v, int suc = 1) {
  for (int i = 0; i < v.size(); i++)
    print(v[i], i == (int)(v.size()) - 1 ? suc : 2);
}
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
const int maxn = 3e5 + 7;
#define pii pair<int, int>

pii a[maxn];
pii merge(pii a, pii b) {
  pii c;
  if (a.first < b.first)
    swap(a, b);
  c.first = a.first;
  c.second = max(a.second, b.first);
  return c;
}
int main() {
  int n;
  cin >> n;
  int pow2 = (1 << n);
  for (int i = 0, x; i < pow2; ++i) {
    cin >> x;
    a[i] = {x, 0};
  }
  for (int j = 0; j < n; ++j) {
    for (int i = 0; i < pow2; ++i) {
      if ((i >> j) & 1)
        a[i] = merge(a[i], a[i ^ (1 << j)]);
    }
  }
  int ans = 0;
  for (int i = 1; i < pow2; ++i) {
    print(max(ans, a[i].first + a[i].second));
    ans = max(ans, a[i].first + a[i].second);
  }
}