Problem p03519 (C++)

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

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a) begin((a)), end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)

#define FF first
#define SS second

#define DUMP(x) cout << #x << ":" << (x) << endl
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.FF >> p.SS;
}
template <class T> istream &operator>>(istream &is, vector<T> &xs) {
  for (auto &x : xs)
    is >> x;
  return is;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << p.FF << " " << p.SS;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &xs) {
  for (unsigned int i = 0; i < xs.size(); ++i)
    os << (i ? " " : "") << xs[i];
  return os;
}
template <class T> void maxi(T &x, T y) {
  if (x < y)
    x = y;
}
template <class T> void mini(T &x, T y) {
  if (x > y)
    x = y;
}

const double EPS = 1e-10;
const double PI = acos(-1.0);
const LL MOD = 1e9 + 7;

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  LL N, Q;
  cin >> N >> Q;

  VL as(N * 2), bs(N * 2);
  cin >> as >> bs;

  if (N == 1) {
    while (Q--) {
      int p, x, y;
      cin >> p >> x >> y;
      --p;
      as[p] = x;
      bs[p] = y;
      cout << as[0] + as[1] << endl;
    }
    return 0;
  }

  int par = 0;
  LL sum = as[0] + as.back();
  multiset<LL> dels;
  FOR(i, 1, 2 * N - 1) {
    LL mx = max(as[i], bs[i]);
    LL mn = min(as[i], bs[i]);
    sum += mx;
    if (mx == as[i])
      par ^= 1;
    dels.insert(mn - mx);
  }

  while (Q--) {
    LL p, x, y;
    cin >> p >> x >> y;
    --p;

    LL mx = max(as[p], bs[p]);
    LL mn = min(as[p], bs[p]);
    sum -= mx;
    if (mx == as[p])
      par ^= 1;
    auto it = dels.find(mn - mx);
    dels.erase(it);

    as[p] = x;
    bs[p] = y;
    LL mx2 = max(as[p], bs[p]);
    LL mn2 = min(as[p], bs[p]);
    sum += mx2;
    if (mx2 == as[p])
      par ^= 1;
    dels.insert(mn2 - mx2);

    LL ans = 0;
    if (par == 0) {
      ans = sum;
    } else {
      auto it = dels.rbegin();
      assert(it != dels.rend());
      ans = sum + *it;
    }
    cout << ans << endl;
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a) begin((a)), end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)

#define FF first
#define SS second

#define DUMP(x) cout << #x << ":" << (x) << endl
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.FF >> p.SS;
}
template <class T> istream &operator>>(istream &is, vector<T> &xs) {
  for (auto &x : xs)
    is >> x;
  return is;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << p.FF << " " << p.SS;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &xs) {
  for (unsigned int i = 0; i < xs.size(); ++i)
    os << (i ? " " : "") << xs[i];
  return os;
}
template <class T> void maxi(T &x, T y) {
  if (x < y)
    x = y;
}
template <class T> void mini(T &x, T y) {
  if (x > y)
    x = y;
}

const double EPS = 1e-10;
const double PI = acos(-1.0);
const LL MOD = 1e9 + 7;

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  LL N, Q;
  cin >> N >> Q;

  VL as(N * 2), bs(N * 2);
  cin >> as >> bs;

  if (N == 1) {
    while (Q--) {
      int p, x, y;
      cin >> p >> x >> y;
      --p;
      as[p] = x;
      bs[p] = y;
      cout << as[0] + as[1] << endl;
    }
    return 0;
  }

  int par = 0;
  LL sum = as[0] + as.back();
  multiset<LL> dels;
  FOR(i, 1, 2 * N - 1) {
    LL mx = max(as[i], bs[i]);
    LL mn = min(as[i], bs[i]);
    sum += mx;
    if (mx == as[i])
      par ^= 1;
    dels.insert(mn - mx);
  }

  while (Q--) {
    LL p, x, y;
    cin >> p >> x >> y;
    --p;
    if (p == 0 || p == 2 * N - 1) {
      sum -= as[p] - x;
      as[p] = x;
      cout << sum + (par ? *dels.rbegin() : 0ll) << endl;
      continue;
    }
    LL mx = max(as[p], bs[p]);
    LL mn = min(as[p], bs[p]);
    sum -= mx;
    if (mx == as[p])
      par ^= 1;
    auto it = dels.find(mn - mx);
    dels.erase(it);

    as[p] = x;
    bs[p] = y;
    LL mx2 = max(as[p], bs[p]);
    LL mn2 = min(as[p], bs[p]);
    sum += mx2;
    if (mx2 == as[p])
      par ^= 1;
    dels.insert(mn2 - mx2);

    LL ans = 0;
    if (par == 0) {
      ans = sum;
    } else {
      auto it = dels.rbegin();
      assert(it != dels.rend());
      ans = sum + *it;
    }
    cout << ans << endl;
  }

  return 0;
}