Problem p03202 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
template <class T> using Vec2 = vector<vector<T>>;
#define REP(i, m, n) for (ll i = (m); i < (n); ++i)
#define REPN(i, m, n) for (ll i = (m); i <= (n); ++i)
#define REPR(i, m, n) for (ll i = (m)-1; i >= (n); --i)
#define REPNR(i, m, n) for (ll i = (m); i >= (n); --i)
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define fs first
#define sc second
template <class T1, class T2> bool chmax(T1 &a, const T2 b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T1, class T2> bool chmin(T1 &a, const T2 b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
ll pow2(const int n) { return (1LL << n); }
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (const T &i : v)
    os << i << ' ';
  return os;
}
void co() { cout << '\n'; }
template <class Head, class... Tail> void co(Head &&head, Tail &&...tail) {
  cout << head << ' ';
  co(forward<Tail>(tail)...);
}
void ce() { cerr << '\n'; }
template <class Head, class... Tail> void ce(Head &&head, Tail &&...tail) {
  cerr << head << ' ';
  ce(forward<Tail>(tail)...);
}
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
}
void setp(const int n) { cout << fixed << setprecision(n); }
constexpr int INF = 1000000001;
constexpr ll LINF = 1000000000000000001;
constexpr ll MOD = 1000000007;
constexpr ll MOD_N = 998244353;
constexpr ld EPS = 1e-11;
const double PI = acos(-1);

template <class T> struct BIT {
  ll N;
  vector<T> data;

  BIT(int64_t n) { init(n); }

  void init(int64_t n) {
    N = 1;
    while (N <= n)
      N <<= 1;
    data.assign(N, 0);
  }

  void build(vector<T> v) {
    data.assign(N, 0);
    for (size_t i = 0; i < v.size(); ++i) {
      if (i == 0)
        trans(i, v[i]);
      else
        trans(i, v[i] - v[i - 1]);
    }
    trans(v.size(), -v.back());
  }

  void trans(int64_t k, T x) {
    ++k;
    while (k <= N) {
      data[k] += x;
      k += k & -k;
    }
  }

  void update(int64_t k, T x) { add(k, k + 1, x - at(k)); }

  void add(int64_t k, T x) { add(k, k + 1, x); }
  void add(int64_t a, int64_t b, T x) {
    trans(a, x);
    trans(b, -x);
  }

  T at(int64_t k) {
    ++k;
    T res = 0;
    while (k > 0) {
      res += data[k];
      k -= k & -k;
    }
    return res;
  }
};

int main(void) {
  ll n;
  cin >> n;
  Vec a(n);
  rep(i, n) cin >> a[i];
  rep(i, n) chmin(a[i], 3e5);

  ll l = 0, r = 3e5;
  while (r - l > 1) {
    ll m = (l + r) / 2;
    BIT<ll> bit(3e5);
    ll len = 0;
    rep(i, n) {
      if (a[i] > len) {
        len = a[i];
        continue;
      }
      len = a[i];
      if (a[i] == 3e5)
        continue;
      if (bit.at(a[i]) < 0)
        bit.update(a[i], 0);
      bit.add(a[i], 1);
      bit.add(a[i] + 1, 3e5, -INF);
      ll p = a[i];
      while (p >= 1 && bit.at(p) >= m) {
        bit.update(p, 0);
        --p;
        if (bit.at(p) < 0)
          bit.update(p, 0);
        bit.add(p, 1);
      }
    }

    if (bit.at(0) == 0)
      r = m;
    else
      l = m;
  }
  co(r);

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
template <class T> using Vec2 = vector<vector<T>>;
#define REP(i, m, n) for (ll i = (m); i < (n); ++i)
#define REPN(i, m, n) for (ll i = (m); i <= (n); ++i)
#define REPR(i, m, n) for (ll i = (m)-1; i >= (n); --i)
#define REPNR(i, m, n) for (ll i = (m); i >= (n); --i)
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define fs first
#define sc second
template <class T1, class T2> bool chmax(T1 &a, const T2 b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T1, class T2> bool chmin(T1 &a, const T2 b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
ll pow2(const int n) { return (1LL << n); }
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (const T &i : v)
    os << i << ' ';
  return os;
}
void co() { cout << '\n'; }
template <class Head, class... Tail> void co(Head &&head, Tail &&...tail) {
  cout << head << ' ';
  co(forward<Tail>(tail)...);
}
void ce() { cerr << '\n'; }
template <class Head, class... Tail> void ce(Head &&head, Tail &&...tail) {
  cerr << head << ' ';
  ce(forward<Tail>(tail)...);
}
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
}
void setp(const int n) { cout << fixed << setprecision(n); }
constexpr int INF = 1000000001;
constexpr ll LINF = 1000000000000000001;
constexpr ll MOD = 1000000007;
constexpr ll MOD_N = 998244353;
constexpr ld EPS = 1e-11;
const double PI = acos(-1);

template <class T> struct BIT {
  ll N;
  vector<T> data;

  BIT(int64_t n) { init(n); }

  void init(int64_t n) {
    N = 1;
    while (N <= n)
      N <<= 1;
    data.assign(N, 0);
  }

  void build(vector<T> v) {
    data.assign(N, 0);
    for (size_t i = 0; i < v.size(); ++i) {
      if (i == 0)
        trans(i, v[i]);
      else
        trans(i, v[i] - v[i - 1]);
    }
    trans(v.size(), -v.back());
  }

  void trans(int64_t k, T x) {
    ++k;
    while (k <= N) {
      data[k] += x;
      k += k & -k;
    }
  }

  void update(int64_t k, T x) { add(k, k + 1, x - at(k)); }

  void add(int64_t k, T x) { add(k, k + 1, x); }
  void add(int64_t a, int64_t b, T x) {
    trans(a, x);
    trans(b, -x);
  }

  T at(int64_t k) {
    ++k;
    T res = 0;
    while (k > 0) {
      res += data[k];
      k -= k & -k;
    }
    return res;
  }
};

int main(void) {
  ll n;
  cin >> n;
  Vec a(n);
  rep(i, n) cin >> a[i];
  rep(i, n) chmin(a[i], 3e5);

  ll l = 0, r = 3e5;
  while (r - l > 1) {
    ll m = (l + r) / 2;
    BIT<ll> bit(3e5);
    ll len = 0;
    rep(i, n) {
      if (a[i] > len) {
        len = a[i];
        continue;
      }
      len = a[i];
      if (a[i] == 3e5)
        continue;
      if (bit.at(a[i]) < 0)
        bit.update(a[i], 0);
      bit.add(a[i], 1);
      bit.add(a[i] + 1, 3e5, -INF);
      ll p = a[i];
      while (p >= 1 && bit.at(p) >= m) {
        bit.update(p, 0);
        --p;
        if (bit.at(p) < 0)
          bit.update(p, 0);
        bit.add(p, 1);
      }
      if (bit.at(0) != 0)
        break;
    }

    if (bit.at(0) == 0)
      r = m;
    else
      l = m;
  }
  co(r);

  return 0;
}