Problem p02943 (C++)

Buggy submission (status: Time Limit Exceeded):
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
constexpr long long INF = 1LL << 60;
constexpr long long MOD = 1000000007;
double PI = acos(-1.0);
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (n); ++i)
#define rrep(i, n) for (ll i = (n - 1); i >= 0; --i)
#define perm(c)                                                                \
  sort(ALL(c));                                                                \
  for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define ALL(obj) (obj).begin(), (obj).end()
#define RALL(obj) (obj).rbegin(), (obj).rend()
#define pb push_back
#define to_s to_string
#define len(v) (ll) v.size()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define print(x) cout << (x) << '\n'
#define drop(x) cout << (x) << '\n', exit(0)
#define debug(x) cout << #x << ": " << (x) << '\n'
using namespace std;
using ll = long long;
typedef pair<ll, ll> P;
typedef vector<ll> vec;
typedef vector<vector<ll>> vec2;
typedef vector<vector<vector<ll>>> vec3;
template <class S, class T> inline bool chmax(S &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class S, class T> inline bool chmin(S &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
inline ll msb(ll v) { return 1 << (31 - __builtin_clzll(v)); }
inline ll devc(ll x, ll y) { return (x + y - 1) / y; }
inline ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
inline ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &t) {
  os << get<0>(t) << " " << get<1>(t) << " " << get<2>(t);
  return os;
}

template <typename T1, typename T2, typename T3>
istream &operator>>(istream &is, tuple<T1, T2, T3> &t) {
  is >> get<0>(t) >> get<1>(t) >> get<2>(t);
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); ++i) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  int ct = 0;
  for (auto &s : st)
    cout << s << (++ct != st.size() ? " " : "");
  return os;
}

template <typename T>
constexpr set<T> &operator|=(set<T> &st1, const set<T> &st2) {
  for (auto &s : st2)
    st1.insert(s);
  return st1;
}

template <typename T>
constexpr set<T> &operator-=(set<T> &st1, const set<T> &st2) {
  for (auto &s : st2)
    if (st1.count(s))
      st1.erase(s);
  return st1;
}

template <typename T>
constexpr set<T> &operator&=(set<T> &st1, const set<T> &st2) {
  auto itr = st1.begin();
  while (itr != st1.end()) {
    if (!st2.count(*itr))
      itr = st1.erase(itr);
    else
      ++itr;
  }
  return st1;
}

template <typename T>
constexpr set<T> operator|(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res |= st2;
  return res;
}

template <typename T>
constexpr set<T> operator-(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res -= st2;
  return res;
}

template <typename T>
constexpr set<T> operator&(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res &= st2;
  return res;
}

/*---------------------------------    Tools
 * ------------------------------------------*/
template <typename T> vector<T> cumsum(const vector<T> &X) {
  vector<T> res(X.size() + 1, 0);
  for (int i = 0; i < X.size(); ++i)
    res[i + 1] += res[i] + X[i];
  return res;
}

template <typename S, typename T, typename F>
pair<T, T> bisearch(S left, T right, F f) {
  while (abs(right - left) > 1) {
    T mid = (right + left) / 2;
    if (f(mid))
      right = mid;
    else
      left = mid;
  }
  return {left, right};
}

template <typename S, typename T, typename F>
double trisearch(S left, T right, F f, int maxLoop = 90) {
  double low = left, high = right;
  while (maxLoop--) {
    double mid_left = high / 3 + low * 2 / 3;
    double mid_right = high * 2 / 3 + low / 3;
    if (f(mid_left) >= f(mid_right))
      low = mid_left;
    else
      high = mid_right;
  }
  return (low + high) * 0.5;
}

template <typename F> ll ternarySearch(ll L, ll R, F f) { //[L, R)
  ll lo = L - 1, hi = R - 1;
  while (lo + 1 != hi) {
    ll mi = (lo + hi) / 2;
    if (f(mi) <= f(mi + 1))
      hi = mi;
    else
      lo = mi;
  }
  return hi;
}

/*------------------------------- Main Code Here
 * -----------------------------------------*/

int main() {
  ll N, K;
  cin >> N >> K;
  string S;
  cin >> S;

  string now = S;
  chmin(K, 2 * N);
  while (K--) {
    string tmp = now;
    reverse(ALL(now));
    tmp += now;

    if (K)
      reverse(ALL(tmp));

    string comp(N, 'z');

    rep(i, 2 * N) if (i + N < 2 * N) {
      string x = tmp.substr(i, N);
      chmin(comp, x);
    }
    if (K)
      reverse(ALL(comp));
    now = comp;
  }

  print(now);

  return 0;
}

Fixed submission:
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
constexpr long long INF = 1LL << 60;
constexpr long long MOD = 1000000007;
double PI = acos(-1.0);
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (n); ++i)
#define rrep(i, n) for (ll i = (n - 1); i >= 0; --i)
#define perm(c)                                                                \
  sort(ALL(c));                                                                \
  for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define ALL(obj) (obj).begin(), (obj).end()
#define RALL(obj) (obj).rbegin(), (obj).rend()
#define pb push_back
#define to_s to_string
#define len(v) (ll) v.size()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define print(x) cout << (x) << '\n'
#define drop(x) cout << (x) << '\n', exit(0)
#define debug(x) cout << #x << ": " << (x) << '\n'
using namespace std;
using ll = long long;
typedef pair<ll, ll> P;
typedef vector<ll> vec;
typedef vector<vector<ll>> vec2;
typedef vector<vector<vector<ll>>> vec3;
template <class S, class T> inline bool chmax(S &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class S, class T> inline bool chmin(S &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
inline ll msb(ll v) { return 1 << (31 - __builtin_clzll(v)); }
inline ll devc(ll x, ll y) { return (x + y - 1) / y; }
inline ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
inline ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &t) {
  os << get<0>(t) << " " << get<1>(t) << " " << get<2>(t);
  return os;
}

template <typename T1, typename T2, typename T3>
istream &operator>>(istream &is, tuple<T1, T2, T3> &t) {
  is >> get<0>(t) >> get<1>(t) >> get<2>(t);
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); ++i) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  int ct = 0;
  for (auto &s : st)
    cout << s << (++ct != st.size() ? " " : "");
  return os;
}

template <typename T>
constexpr set<T> &operator|=(set<T> &st1, const set<T> &st2) {
  for (auto &s : st2)
    st1.insert(s);
  return st1;
}

template <typename T>
constexpr set<T> &operator-=(set<T> &st1, const set<T> &st2) {
  for (auto &s : st2)
    if (st1.count(s))
      st1.erase(s);
  return st1;
}

template <typename T>
constexpr set<T> &operator&=(set<T> &st1, const set<T> &st2) {
  auto itr = st1.begin();
  while (itr != st1.end()) {
    if (!st2.count(*itr))
      itr = st1.erase(itr);
    else
      ++itr;
  }
  return st1;
}

template <typename T>
constexpr set<T> operator|(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res |= st2;
  return res;
}

template <typename T>
constexpr set<T> operator-(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res -= st2;
  return res;
}

template <typename T>
constexpr set<T> operator&(const set<T> &st1, const set<T> &st2) {
  set<T> res = st1;
  res &= st2;
  return res;
}

/*---------------------------------    Tools
 * ------------------------------------------*/
template <typename T> vector<T> cumsum(const vector<T> &X) {
  vector<T> res(X.size() + 1, 0);
  for (int i = 0; i < X.size(); ++i)
    res[i + 1] += res[i] + X[i];
  return res;
}

template <typename S, typename T, typename F>
pair<T, T> bisearch(S left, T right, F f) {
  while (abs(right - left) > 1) {
    T mid = (right + left) / 2;
    if (f(mid))
      right = mid;
    else
      left = mid;
  }
  return {left, right};
}

template <typename S, typename T, typename F>
double trisearch(S left, T right, F f, int maxLoop = 90) {
  double low = left, high = right;
  while (maxLoop--) {
    double mid_left = high / 3 + low * 2 / 3;
    double mid_right = high * 2 / 3 + low / 3;
    if (f(mid_left) >= f(mid_right))
      low = mid_left;
    else
      high = mid_right;
  }
  return (low + high) * 0.5;
}

template <typename F> ll ternarySearch(ll L, ll R, F f) { //[L, R)
  ll lo = L - 1, hi = R - 1;
  while (lo + 1 != hi) {
    ll mi = (lo + hi) / 2;
    if (f(mi) <= f(mi + 1))
      hi = mi;
    else
      lo = mi;
  }
  return hi;
}

/*------------------------------- Main Code Here
 * -----------------------------------------*/

int main() {
  ll N, K;
  cin >> N >> K;
  string S;
  cin >> S;

  string now = S;
  chmin(K, 10 + log2(N));
  while (K--) {
    string tmp = now;
    reverse(ALL(now));
    tmp += now;

    if (K)
      reverse(ALL(tmp));

    string comp(N, 'z');

    rep(i, 2 * N) if (i + N < 2 * N) {
      string x = tmp.substr(i, N);
      chmin(comp, x);
    }
    if (K)
      reverse(ALL(comp));
    now = comp;
  }

  print(now);

  return 0;
}