Problem p03202 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using VVV = vector<VV>;
using VVVll = vector<VVll>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) (a).begin(), (a).end()
struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline int sz(T &x) { return x.size(); }
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << ' ';
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int fLog2(const int x) {
  assert(x > 0);
  return 31 - __builtin_clz(x);
} // floor(log2(x))
inline int fLog2(const ll x) {
  assert(x > 0);
  return 63 - __builtin_clzll(x);
}
inline int cLog2(const int x) {
  assert(x > 0);
  return (x == 1) ? 0 : 32 - __builtin_clz(x - 1);
} // ceil(log2(x))
inline int cLog2(const ll x) {
  assert(x > 0);
  return (x == 1) ? 0 : 64 - __builtin_clzll(x - 1);
}
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
void fail() {
  cout << -1 << '\n';
  exit(0);
}
// const int INF  = 1<<30;
// const ll INFll = 1ll<<62;
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
// const int MOD  = int(1e9)+7;
// const int MOD  = 998244353;

using VP = vector<P>;

bool inc(VP &v, int k) {
  auto [x, y] = v.back();
  // assert(0 <= x && x < k && 0 < y);
  if (x < k - 1) {
    --v.back().second;
    if (v.back().second == 0)
      v.pop_back();
    if (v.back().first == x + 1)
      ++v.back().second;
    else
      v.emplace_back(x + 1, 1);
    return true;
  }
  if (sz(v) == 1)
    return false;
  v.pop_back();
  // assert(inc(v, k));
  inc(v, k);
  v.emplace_back(0, y);
  return true;
}

int main() {
  int n;
  cin >> n;
  V a(n);
  cin >> a;

  auto f = [&](int k) {
    VP v = {{0, a[0]}};
    // cout << v << '\n';

    rep2(i, 1, n) {
      if (a[i - 1] == a[i]) {
        if (!inc(v, k))
          return false;
      } else if (a[i - 1] < a[i]) {
        auto [x, y] = v.back();
        if (x == 0)
          v.back().second += a[i] - a[i - 1];
        else
          v.emplace_back(0, a[i] - a[i - 1]);
      } else {
        int d = a[i - 1] - a[i];
        while (d) {
          if (v.back().second > d) {
            v.back().second -= d;
            d = 0;
          } else {
            d -= v.back().second;
            v.pop_back();
          }
        }
        if (!inc(v, k))
          return false;
      }
      // cout << v << '\n';
    }
    return true;
  };

  ll ok = ll(1e9) + 5;
  ll ng = 0;
  while (llabs(ok - ng) > 1) {
    ll md = (ok + ng) >> 1;
    if (f(md))
      ok = md;
    else
      ng = md;
  }

  ll ans = ok;
  cout << ans << '\n';
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using VVV = vector<VV>;
using VVVll = vector<VVll>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) (a).begin(), (a).end()
struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline int sz(T &x) { return x.size(); }
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << ' ';
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int fLog2(const int x) {
  assert(x > 0);
  return 31 - __builtin_clz(x);
} // floor(log2(x))
inline int fLog2(const ll x) {
  assert(x > 0);
  return 63 - __builtin_clzll(x);
}
inline int cLog2(const int x) {
  assert(x > 0);
  return (x == 1) ? 0 : 32 - __builtin_clz(x - 1);
} // ceil(log2(x))
inline int cLog2(const ll x) {
  assert(x > 0);
  return (x == 1) ? 0 : 64 - __builtin_clzll(x - 1);
}
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
void fail() {
  cout << -1 << '\n';
  exit(0);
}
// const int INF  = 1<<30;
// const ll INFll = 1ll<<62;
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
// const int MOD  = int(1e9)+7;
// const int MOD  = 998244353;

using VP = vector<P>;

bool inc(VP &v, int k) {
  auto [x, y] = v.back();
  // assert(0 <= x && x < k && 0 < y);
  if (x < k - 1) {
    --v.back().second;
    if (v.back().second == 0)
      v.pop_back();
    if (sz(v) && v.back().first == x + 1)
      ++v.back().second;
    else
      v.emplace_back(x + 1, 1);
    return true;
  }
  if (sz(v) == 1)
    return false;
  v.pop_back();
  // assert(inc(v, k));
  inc(v, k);
  v.emplace_back(0, y);
  return true;
}

int main() {
  int n;
  cin >> n;
  V a(n);
  cin >> a;

  auto f = [&](int k) {
    VP v = {{0, a[0]}};
    // cout << v << '\n';

    rep2(i, 1, n) {
      if (a[i - 1] == a[i]) {
        if (!inc(v, k))
          return false;
      } else if (a[i - 1] < a[i]) {
        auto [x, y] = v.back();
        if (x == 0)
          v.back().second += a[i] - a[i - 1];
        else
          v.emplace_back(0, a[i] - a[i - 1]);
      } else {
        int d = a[i - 1] - a[i];
        while (d) {
          if (v.back().second > d) {
            v.back().second -= d;
            d = 0;
          } else {
            d -= v.back().second;
            v.pop_back();
          }
        }
        if (!inc(v, k))
          return false;
      }
      // cout << v << '\n';
    }
    return true;
  };

  ll ok = ll(1e9) + 5;
  ll ng = 0;
  while (llabs(ok - ng) > 1) {
    ll md = (ok + ng) >> 1;
    if (f(md))
      ok = md;
    else
      ng = md;
  }

  ll ans = ok;
  cout << ans << '\n';
  return 0;
}