Problem p03187 (C++)

Buggy submission (status: Runtime Error):
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

using ll = long long;

template <typename T> using PQ = priority_queue<T>;
template <typename T> using GPQ = priority_queue<T, vector<T>, greater<T>>;

#define mp make_pair
#define mt make_tuple

/* ----------- debug ---------- */

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p);

template <class T> ostream &operator<<(ostream &os, vector<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class T> ostream &operator<<(ostream &os, set<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p) {
  return os << "(" << p.first << "," << p.second << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1000000007;
// const ll MOD = 998244353;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T> inline T sq(T a) { return a * a; }

template <typename T> T gcd(T a, T b) {
  if (a > b)
    return gcd(b, a);
  return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U> T mypow(T b, U n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return b /* % MOD */;
  if (n % 2 == 0) {
    return mypow(b * b /* % MOD */, n / 2);
  } else {
    return mypow(b, n - 1) * b /* % MOD */;
  }
}

ll pcnt(ll b) { return __builtin_popcountll(b); }

template <typename T> T iceil(T n, T d) { return (n + d - 1) / d; }

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {
  ll L;
  int N;
  cin >> L >> N;

  if (N > 2000)
    terminate();

  ll X[N + 2];
  X[0] = 0;
  for (int i = 1; i <= N; ++i) {
    cin >> X[i];
  }
  X[N + 1] = L;

  ll cost = 0;
  ll ans = max(ans, L - X[N]);

  for (int l = 1; l <= N - l; ++l) {
    // X[1] ~ X[l]までのl個
    // X[l+1] ~ X[2l]までのl個

    cost += X[l];
    cost -= L - X[l];
    cost += (L - X[l * 2 - 1]) + (L - X[l * 2]);

    ans = max(ans, cost * 2 - min(X[l], L - X[l + 1] - (L - X[l * 2 + 1]) * 2));
  }

  for (int i = 0; i <= N + 1; ++i) {
    X[i] = L - X[i];
  }

  reverse(X, X + N + 2);

  cost = 0;
  ans = max(ans, L - X[N]);

  for (int l = 1; l <= N - l; ++l) {
    // X[1] ~ X[l]までのl個
    // X[l+1] ~ X[2l]までのl個

    cost += X[l];
    cost -= L - X[l];
    cost += (L - X[l * 2 - 1]) + (L - X[l * 2]);

    ans = max(ans, cost * 2 - min(X[l], L - X[l + 1] - (L - X[l * 2 + 1]) * 2));
  }

  cout << ans << endl;

  return 0;
}


Fixed submission:
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

using ll = long long;

template <typename T> using PQ = priority_queue<T>;
template <typename T> using GPQ = priority_queue<T, vector<T>, greater<T>>;

#define mp make_pair
#define mt make_tuple

/* ----------- debug ---------- */

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p);

template <class T> ostream &operator<<(ostream &os, vector<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class T> ostream &operator<<(ostream &os, set<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p) {
  return os << "(" << p.first << "," << p.second << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1000000007;
// const ll MOD = 998244353;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T> inline T sq(T a) { return a * a; }

template <typename T> T gcd(T a, T b) {
  if (a > b)
    return gcd(b, a);
  return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U> T mypow(T b, U n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return b /* % MOD */;
  if (n % 2 == 0) {
    return mypow(b * b /* % MOD */, n / 2);
  } else {
    return mypow(b, n - 1) * b /* % MOD */;
  }
}

ll pcnt(ll b) { return __builtin_popcountll(b); }

template <typename T> T iceil(T n, T d) { return (n + d - 1) / d; }

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {
  ll L;
  int N;
  cin >> L >> N;

  ll X[N + 2];
  X[0] = 0;
  for (int i = 1; i <= N; ++i) {
    cin >> X[i];
  }
  X[N + 1] = L;

  ll cost = 0;
  ll ans = max(ans, L - X[N]);

  for (int l = 1; l <= N - l; ++l) {
    // X[1] ~ X[l]までのl個
    // X[l+1] ~ X[2l]までのl個

    cost += X[l];
    cost -= L - X[l];
    cost += (L - X[l * 2 - 1]) + (L - X[l * 2]);

    ans = max(ans, cost * 2 - min(X[l], L - X[l + 1] - (L - X[l * 2 + 1]) * 2));
  }

  for (int i = 0; i <= N + 1; ++i) {
    X[i] = L - X[i];
  }

  reverse(X, X + N + 2);

  cost = 0;
  ans = max(ans, L - X[N]);

  for (int l = 1; l <= N - l; ++l) {
    // X[1] ~ X[l]までのl個
    // X[l+1] ~ X[2l]までのl個

    cost += X[l];
    cost -= L - X[l];
    cost += (L - X[l * 2 - 1]) + (L - X[l * 2]);

    ans = max(ans, cost * 2 - min(X[l], L - X[l + 1] - (L - X[l * 2 + 1]) * 2));
  }

  cout << ans << endl;

  return 0;
}