Problem p03309 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < (N); i++)
#define rem(i, N, s) for (ll i{N}; i > (s); i--)
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
const int MOD = (int)1e9 + 7;
const string rt = "\n", sp = " ";
/* 最大公約数 */ template <typename T> T gcd(T a, T b) {
  return b != 0 ? gcd(b, a % b) : a;
} /* 最小公倍数 */
template <typename T> T lcm(T a, T b) {
  return a * b / gcd(a, b);
} /* UnionFind */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
}; /* コンビネーション */
ll comb(ll n, ll r) {
  if (n < r)
    return 0;
  if (r * 2 > n)
    r = n - r;
  ll dividend = 1;
  ll divisor = 1;
  for (ll i = 1; i <= r; ++i) {
    dividend *= (n - i + 1);
    divisor *= i;
  }
  return dividend / divisor;
} /* MODコンビネーション */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
} /* 多次元ベクター */
template <class T> vector<T> mvec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto mvec(size_t a, Ts... ts) {
  return vector<decltype(mvec<T>(ts...))>(a, mvec<T>(ts...));
} /* 多次元デック */
template <class T> deque<T> mdeq(size_t a) { return deque<T>(a); }
template <class T, class... Ts> auto mdeq(size_t a, Ts... ts) {
  return deque<decltype(mdeq<T>(ts...))>(a, mdeq<T>(ts...));
} /* テスト関数 */
void test(ll n) { cout << "test" << n << endl; } /* 小数点以下 */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) {
  cout << defaultfloat << setprecision(n);
} /* 重み付きUnionFind */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
}; /* 約数列挙 */
deque<ll> divisor(ll n) {
  deque<ll> ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));
  return (ret);
} /* 最頻値 */
template <typename T> auto mode(T data) {
  if (data.size() == 1)
    return *begin(data);
  sort(begin(data), end(data));
  typename decltype(data)::value_type mode{};
  size_t n{}, count{1};
  for (auto iter = adjacent_find(begin(data), end(data)), last = end(data),
            next = end(data);
       iter != last;) {
    next = upper_bound(iter, last, *iter);
    count = distance(iter, next);
    if (n < count)
      n = count, mode = *iter;
    iter = adjacent_find(next, last);
  }
  return mode;
} /* 中央値 */
template <typename T> auto median(T data) {
  sort(begin(data), end(data));
  size_t median_index = data.size() / 2;
  return (
      data.size() % 2 == 0
          ? static_cast<double>(data[median_index] + data[median_index - 1]) / 2
          : data[median_index]);
} /* IO高速化 */
void fastio() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
}

/* ここからコード開始 */

int main() {
  fastio;
  ll N;
  cin >> N;
  auto A = mdeq<ll>(N);
  rep(i, 0, N) {
    cin >> A[i];
    A[i] -= i;
  }
  ll m = median(A);
  ll ans{};
  rep(i, 0, N) { ans += abs(A[i] - median(A)); }
  cout << ans << rt;
}

Fixed submission:
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < (N); i++)
#define rem(i, N, s) for (ll i{N}; i > (s); i--)
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
const int MOD = (int)1e9 + 7;
const string rt = "\n", sp = " ";
/* 最大公約数 */ template <typename T> T gcd(T a, T b) {
  return b != 0 ? gcd(b, a % b) : a;
} /* 最小公倍数 */
template <typename T> T lcm(T a, T b) {
  return a * b / gcd(a, b);
} /* UnionFind */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
}; /* コンビネーション */
ll comb(ll n, ll r) {
  if (n < r)
    return 0;
  if (r * 2 > n)
    r = n - r;
  ll dividend = 1;
  ll divisor = 1;
  for (ll i = 1; i <= r; ++i) {
    dividend *= (n - i + 1);
    divisor *= i;
  }
  return dividend / divisor;
} /* MODコンビネーション */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
} /* 多次元ベクター */
template <class T> vector<T> mvec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto mvec(size_t a, Ts... ts) {
  return vector<decltype(mvec<T>(ts...))>(a, mvec<T>(ts...));
} /* 多次元デック */
template <class T> deque<T> mdeq(size_t a) { return deque<T>(a); }
template <class T, class... Ts> auto mdeq(size_t a, Ts... ts) {
  return deque<decltype(mdeq<T>(ts...))>(a, mdeq<T>(ts...));
} /* テスト関数 */
void test(ll n) { cout << "test" << n << endl; } /* 小数点以下 */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) {
  cout << defaultfloat << setprecision(n);
} /* 重み付きUnionFind */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
}; /* 約数列挙 */
deque<ll> divisor(ll n) {
  deque<ll> ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));
  return (ret);
} /* 最頻値 */
template <typename T> auto mode(T data) {
  if (data.size() == 1)
    return *begin(data);
  sort(begin(data), end(data));
  typename decltype(data)::value_type mode{};
  size_t n{}, count{1};
  for (auto iter = adjacent_find(begin(data), end(data)), last = end(data),
            next = end(data);
       iter != last;) {
    next = upper_bound(iter, last, *iter);
    count = distance(iter, next);
    if (n < count)
      n = count, mode = *iter;
    iter = adjacent_find(next, last);
  }
  return mode;
} /* 中央値 */
template <typename T> auto median(T data) {
  sort(begin(data), end(data));
  size_t median_index = data.size() / 2;
  return (
      data.size() % 2 == 0
          ? static_cast<double>(data[median_index] + data[median_index - 1]) / 2
          : data[median_index]);
} /* IO高速化 */
void fastio() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
}

/* ここからコード開始 */

int main() {
  fastio;
  ll N;
  cin >> N;
  auto A = mdeq<ll>(N);
  rep(i, 0, N) {
    cin >> A[i];
    A[i] -= i;
  }
  ll m = median(A);
  ll ans{};
  rep(i, 0, N) { ans += abs(A[i] - m); }
  cout << ans << rt;
}