Problem p01557 (C++) — buggy submission, verdict: Memory Limit Exceeded

#include <algorithm>
#include <iostream>
#include <map>
#include <queue>
#include <vector>

using namespace std;

#define rep(i, n) for (int i = 0; i < n; i++)

int N;
vector<int> A, B;

map<vector<int>, int> bfs(vector<int> start) {
  map<vector<int>, int> memo;
  queue<vector<int>> Q;
  Q.push(start);
  memo[start] = 0;

  while (!Q.empty()) {
    vector<int> vi = Q.front();
    Q.pop();

    int cost = memo[vi];

    if (cost >= (N - 1) / 2)
      continue;

    for (int i = 0; i < N; i++) {
      for (int j = i + 2; j <= N; j++) {
        vector<int> tmp = vi;
        reverse(tmp.begin() + i, tmp.begin() + j);
        if (memo.find(tmp) == memo.end()) {
          memo[tmp] = cost + 1;
          Q.push(tmp);
        }
      }
    }
  }

  return memo;
}

int solve() {
  int ans = N - 1;
  map<vector<int>, int> memo_A = bfs(A);
  map<vector<int>, int> memo_B = bfs(B);
  if (memo_A.find(B) != memo_A.end())
    ans = memo_A[B];
  else {
    do {
      if (memo_A.find(B) == memo_A.end())
        continue;
      if (memo_B.find(B) == memo_B.end())
        continue;
      ans = min(ans, memo_A[B] + memo_B[B]);
    } while (next_permutation(B.begin(), B.end()));
  }
  return ans;
}

int main() {
  cin >> N;
  rep(i, N) {
    int a;
    cin >> a;
    A.push_back(a);
  }
  B = A;
  sort(B.begin(), B.end());
  cout << solve() << endl;
  return 0;
}

Error: MLE

Fixed submission:

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

#ifdef _WIN32
#define scanfll(x) scanf("%I64d", x)
#define printfll(x) printf("%I64d", x)
#else
#define scanfll(x) scanf("%lld", x)
#define printfll(x) printf("%lld", x)
#endif
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define repi(i, a, b)                                                          \\
  for (long long i = (long long)(a); i < (long long)(b); i++)
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second
#define mt make_tuple
#define mp make_pair
template <class T1, class T2> bool chmin(T1 &a, T2 b) {
  return b < a && (a = b, true);
}
template <class T1, class T2> bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vll>;
using vld = vector<ld>;
using vi = vector<int>;
using vvi = vector<vi>;
vll conv(vi &v) {
  vll r(v.size());
  rep(i, v.size()) r[i] = v[i];
  return r;
}
using P = pair<ll, ll>;

template <typename T, typename U>
ostream &operator<<(ostream &o, const pair<T, U> &v) {
  o << "(" << v.first << ", " << v.second << ")";
  return o;
}
template <size_t...> struct seq {};
template <size_t N, size_t... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> {};
template <size_t... Is> struct gen_seq<0, Is...> : seq<Is...> {};
template <class Ch, class Tr, class Tuple, size_t... Is>
void print_tuple(basic_ostream<Ch, Tr> &os, Tuple const &t, seq<Is...>) {
  using s = int[];
  (void)s{0, (void(os << (Is == 0 ? "" : ", ") << get<Is>(t)), 0)...};
}
template <class Ch, class Tr, class... Args>
auto operator<<(basic_ostream<Ch, Tr> &os, tuple<Args...> const &t)
    -> basic_ostream<Ch, Tr> & {
  os << "(";
  print_tuple(os, t, gen_seq<sizeof...(Args)>());
  return os << ")";
}
ostream &operator<<(ostream &o, const vvll &v) {
  rep(i, v.size()) {
    rep(j, v[i].size()) o << v[i][j] << " ";
    cout << endl;
  }
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  o << '[';
  rep(i, v.size()) o << v[i] << (i != v.size() - 1 ? ", " : "");
  o << "]";
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const set<T> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U>
ostream &operator<<(ostream &o, const map<T, U> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U>
ostream &operator<<(ostream &o, const unordered_map<T, U> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it;
  o << "]";
  return o;
}
void printbits(ll mask, ll n) {
  rep(i, n) { cout << !!(mask & (1ll << i)); }
  cout << endl;
}
#define ldout fixed << setprecision(40)

static const double EPS = 1e-14;
static const long long INF = 1e18;
static const long long mo = 1e9 + 7;

void ennumerate(unordered_map<string, int> &b, string s) {
  b.clear();

  queue<string> q;
  q.push(s);
  b[s] = 0;
  while (!q.empty()) {
    string t = q.front();
    q.pop();
    //        cout << "###" << endl;
    //        cout << t << endl;
    int next_step = b[t] + 1;
    rep(i, t.size()) repi(j, i + 2, t.size() + 1) {
      reverse(t.begin() + i, t.begin() + j);
      if (!b.count(t)) {
        //                cout << t << endl;
        b[t] = next_step;
        //                cout << next_step << endl;
        q.push(t);
      }
      reverse(t.begin() + i, t.begin() + j);
    }
  }
}
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n;
  cin >> n;
  string org(n, ' ');
  rep(i, org.size()) { org[i] = i + 1 + '0'; }
  string a(n, ' ');
  rep(i, a.size()) {
    int tmp;
    cin >> tmp;
    a[i] = tmp + '0';
  }

  unordered_map<string, int> b1, b2;
  b1.reserve(1000000);
  b2.reserve(1000000);
  ennumerate(b1, org);
  ennumerate(b2, a);
  //    cout << b1 << endl;

  ll ret = n - 1;
  for (auto x : b1) {
    if (b2.count(x.fi)) {
      if (ret > x.se + b2[x.fi]) {
        ret = x.se + b2[x.fi];
        //                cout << x << " " <<  b2[x.fi] << endl;
      }
    }
  }
  cout << ret << endl;
  return 0;
}