Problem p02618 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <algorithm>
#include <array>
#include <chrono>
#include <iostream>
#include <set>
#include <vector>

int const INF = 1e9;

uint64_t xor64(void) {
  static uint64_t x = 88172645463325252ULL;
  x = x ^ (x << 7);
  return x = x ^ (x >> 9);
}

auto b_cl = std::chrono::system_clock::now();
inline double get_time() {
  auto e_cl = std::chrono::system_clock::now();
  double msec =
      std::chrono::duration_cast<std::chrono::milliseconds>(e_cl - b_cl)
          .count();
  return msec;
}

struct state {
  int score_k;
  int type;
  int last[26];
  int score;
  std::vector<state>::iterator prev;
  bool operator<(state const &rhs) const {
    return score_k > rhs.score_k;
  } // reverse order
};

int main() {
  std::cin.tie(0);
  std::ios::sync_with_stdio(false);
  int D;
  std::cin >> D;
  int c[26];
  for (auto &ci : c)
    std::cin >> ci;
  std::vector<std::array<int, 26>> s(D);
  for (int d = 0; d < D; ++d) {
    for (auto &si : s[d])
      std::cin >> si;
  }
  int const B = 750;
  int const K = 3;
  std::vector<int> t(D);

  std::vector<std::vector<state>> states(D + 1);
  auto beam_step = [&](int d, int k) {
    auto &prev = states[d];
    auto &curr = states[d + 1];
    curr.reserve(prev.size() * 26);
    int dd = std::min(d + k, D - 1);
    for (auto it = prev.begin(); it != prev.end(); ++it) {
      for (int T = 0; T < 26; ++T) {
        curr.push_back(*it);
        state &st = curr.back();
        st.prev = it;
        st.last[T] = d;
        st.type = T;
        st.score_k = st.score + s[d][T];
        for (int cx = 0; cx < 26; ++cx) {
          st.score_k -= c[cx] * (d + dd - 2 * st.last[cx]) * (dd - d + 1) / 2;
        }
      }
    }
    std::sort(curr.begin(), curr.end());
    if (curr.size() > B)
      curr.resize(B);
    for (auto &st : curr) {
      st.score += s[d][st.type];
      for (int cx = 0; cx < 26; ++cx) {
        st.score -= c[cx] * (d - st.last[cx]);
      }
    }
  };

  auto beam_search = [&](int k) {
    states.assign(D + 1, std::vector<state>());
    states[0].emplace_back();
    states[0][0].score = 0;
    states[0][0].score_k = 0;
    std::fill_n(states[0][0].last, 26, -1);
    for (int d = 0; d < D; ++d)
      beam_step(d, k);
    auto it = states[D].begin();
    for (int d = D - 1; d >= 0; it = it->prev, --d) {
      t[d] = it->type;
    }
  };
  beam_search(K);
  for (auto ti : t)
    std::cout << ti + 1 << "\\n";
  return 0;
}


Error: TLE

Fixed submission:

#include <algorithm>
#include <array>
#include <chrono>
#include <iostream>
#include <set>
#include <vector>

int const INF = 1e9;

uint64_t xor64(void) {
  static uint64_t x = 88172645463325252ULL;
  x = x ^ (x << 7);
  return x = x ^ (x >> 9);
}

auto b_cl = std::chrono::system_clock::now();
inline double get_time() {
  auto e_cl = std::chrono::system_clock::now();
  double msec =
      std::chrono::duration_cast<std::chrono::milliseconds>(e_cl - b_cl)
          .count();
  return msec;
}

struct state {
  int score_k;
  int type;
  int last[26];
  int score;
  std::vector<state>::iterator prev;
  bool operator<(state const &rhs) const {
    return score_k > rhs.score_k;
  } // reverse order
};

int main() {
  std::cin.tie(0);
  std::ios::sync_with_stdio(false);
  int D;
  std::cin >> D;
  int c[26];
  for (auto &ci : c)
    std::cin >> ci;
  std::vector<std::array<int, 26>> s(D);
  for (int d = 0; d < D; ++d) {
    for (auto &si : s[d])
      std::cin >> si;
  }
  int const B = 840;
  int const K = 3;
  std::vector<int> t(D);

  std::vector<std::vector<state>> states(D + 1);
  auto beam_step = [&](int d, int k) {
    auto &prev = states[d];
    auto &curr = states[d + 1];
    curr.reserve(prev.size() * 26);
    int dd = std::min(d + k, D - 1);
    for (auto it = prev.begin(); it != prev.end(); ++it) {
      for (int T = 0; T < 26; ++T) {
        curr.push_back(*it);
        state &st = curr.back();
        st.prev = it;
        st.last[T] = d;
        st.type = T;
        st.score_k = st.score + s[d][T];
        for (int cx = 0; cx < 26; ++cx) {
          st.score_k -= c[cx] * (d + dd - 2 * st.last[cx]) * (dd - d + 1) / 2;
        }
      }
    }
    std::sort(curr.begin(), curr.end());
    if (curr.size() > B)
      curr.resize(B);
    for (auto &st : curr) {
      st.score += s[d][st.type];
      for (int cx = 0; cx < 26; ++cx) {
        st.score -= c[cx] * (d - st.last[cx]);
      }
    }
  };

  auto beam_search = [&](int k) {
    states.assign(D + 1, std::vector<state>());
    states[0].emplace_back();
    states[0][0].score = 0;
    states[0][0].score_k = 0;
    std::fill_n(states[0][0].last, 26, -1);
    for (int d = 0; d < D; ++d)
      beam_step(d, k);
  };
  beam_search(K);
  auto it = states[D].begin();
  int ans = it->score;
  for (int d = D - 1; d >= 0; it = it->prev, --d) {
    t[d] = it->type;
  }
  beam_search(K - 1);
  it = states[D].begin();
  if (ans < it->score) {
    for (int d = D - 1; d >= 0; it = it->prev, --d) {
      t[d] = it->type;
    }
  }

  for (auto ti : t)
    std::cout << ti + 1 << "\\n";
  return 0;
}