Problem p03187 (C++)

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

typedef long long signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for (int i = (l); i < (r); i++)
#define incII(i, l, r) for (int i = (l); i <= (r); i++)
#define decID(i, l, r) for (int i = (r)-1; i >= (l); i--)
#define decII(i, l, r) for (int i = (r); i >= (l); i--)
#define inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) < (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define PQ priority_queue

#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define FOR(it, v) for (auto it = v.begin(); it != v.end(); ++it)
#define RFOR(it, v) for (auto it = v.rbegin(); it != v.rend(); ++it)

template <typename T> bool setmin(T &a, T b) {
  if (b < a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmax(T &a, T b) {
  if (b > a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmineq(T &a, T b) {
  if (b <= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmaxeq(T &a, T b) {
  if (b >= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

LL l, n, x[200002], d[200001], s[200002];

LL ID(int l, int r) { return s[r] - s[l]; }
LL CD(int l, int r) { return s[r] - s[l + 1]; }

int main() {
  cin >> l >> n;

  assert(n <= 2000);

  x[0] = 0;
  inc1(i, n) { cin >> x[i]; }
  x[n + 1] = l;

  n++;
  inc(i, n) { d[i] = x[i + 1] - x[i]; }
  inc(i, n) { s[i + 1] = s[i] + d[i]; }

  LL ans = 0, sum = 0;
  inc(i, n) {
    if (i == 0) {
      sum = 0;
    } else {
      if (2 * i - 1 < n) {
        int x = i - 1, y = 2 * i - 1;
        sum += 2 * ID(0, x);
        sum += -2 * CD(x, y);
        sum += 2 * CD(y, n);
        sum += d[x];
      } else {
        int x = 2 * i - 1 - n, y = i - 1;
        sum += -2 * ID(0, x);
        sum += 2 * CD(x, y);
        sum += -2 * CD(y, n);
        sum += d[y];
      }
    }
    setmax(ans, sum);
  }
  inc(i, n) {
    if (i == 0) {
      sum = ID(1, n);
    } else {
      if (2 * i < n) {
        int x = i, y = 2 * i;
        sum += 2 * ID(0, x);
        sum += -2 * CD(x, y);
        sum += 2 * CD(y, n);
        sum += -d[x];
      } else {
        int x = 2 * i - n, y = i;
        sum += -2 * ID(0, x);
        sum += 2 * CD(x, y);
        sum += -2 * CD(y, n);
        sum += -d[y];
      }
    }
    setmax(ans, sum);
  }

  cout << ans << endl;

  return 0;
}


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

typedef long long signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for (int i = (l); i < (r); i++)
#define incII(i, l, r) for (int i = (l); i <= (r); i++)
#define decID(i, l, r) for (int i = (r)-1; i >= (l); i--)
#define decII(i, l, r) for (int i = (r); i >= (l); i--)
#define inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) < (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define PQ priority_queue

#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define FOR(it, v) for (auto it = v.begin(); it != v.end(); ++it)
#define RFOR(it, v) for (auto it = v.rbegin(); it != v.rend(); ++it)

template <typename T> bool setmin(T &a, T b) {
  if (b < a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmax(T &a, T b) {
  if (b > a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmineq(T &a, T b) {
  if (b <= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmaxeq(T &a, T b) {
  if (b >= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

LL l, n, x[200002], d[200001], s[200002];

LL ID(int l, int r) { return s[r] - s[l]; }
LL CD(int l, int r) { return s[r] - s[l + 1]; }

int main() {
  cin >> l >> n;

  x[0] = 0;
  inc1(i, n) { cin >> x[i]; }
  x[n + 1] = l;

  n++;
  inc(i, n) { d[i] = x[i + 1] - x[i]; }
  inc(i, n) { s[i + 1] = s[i] + d[i]; }

  LL ans = 0, sum = 0;
  inc(i, n) {
    if (i == 0) {
      sum = 0;
    } else {
      if (2 * i - 1 < n) {
        int x = i - 1, y = 2 * i - 1;
        sum += 2 * ID(0, x);
        sum += -2 * CD(x, y);
        sum += 2 * CD(y, n);
        sum += d[x];
      } else {
        int x = 2 * i - 1 - n, y = i - 1;
        sum += -2 * ID(0, x);
        sum += 2 * CD(x, y);
        sum += -2 * CD(y, n);
        sum += d[y];
      }
    }
    setmax(ans, sum);
  }
  inc(i, n) {
    if (i == 0) {
      sum = ID(1, n);
    } else {
      if (2 * i < n) {
        int x = i, y = 2 * i;
        sum += 2 * ID(0, x);
        sum += -2 * CD(x, y);
        sum += 2 * CD(y, n);
        sum += -d[x];
      } else {
        int x = 2 * i - n, y = i;
        sum += -2 * ID(0, x);
        sum += 2 * CD(x, y);
        sum += -2 * CD(y, n);
        sum += -d[y];
      }
    }
    setmax(ans, sum);
  }

  cout << ans << endl;

  return 0;
}