Problem p02578 (C++) — buggy submission, verdict: Time Limit Exceeded

/**
 * author: Behradm
 * Created: 2020.08.22 16:40:01
 * In The Name of God
 **/
#include <bits/stdc++.h>

using namespace std;

template <typename A, typename B> string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << '\\n'; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<long long> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  vector<long long> p(n);
  p[0] = a[0];
  for (int i = 1; i < n; i++) {
    p[i] = max(p[i - 1], a[i]);
  }

  long long res = 0;
  for (int i = n - 1; i >= 1; i--) {
    // long long t = Max(0, i + 1);
    // cerr << 0 << " " << i + 1 << " " <
    long long t = p[i];
    if (a[i] < t) {
      res += t - a[i];
      a[i] = t;
    }
  }

  cout << res << '\\n';
  return 0;
}


Error: TLE

Fixed submission:

/**
 * author: Behradm
 * Created: 2020.08.22 16:40:01
 * In The Name of God
 **/
#include <bits/stdc++.h>

using namespace std;

const int N = 20000001;
long long a[N];

long long Max(int L, int R) {
  if (R - L <= 1) {
    return a[L];
  }

  int mid = (L + R) >> 1;
  return max(Max(L, mid), Max(mid, R));
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  long long res = 0;
  for (int i = n - 1; i >= 1; i--) {
    long long t = Max(0, i + 1);
    if (a[i] < t) {
      res += t - a[i];
      a[i] = t;
    }
  }

  cout << res << '\\n';
  return 0;
}