Problem p01551 (C++) — buggy submission, verdict: Memory Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int(i) = 0; (i) < (int)(n); ++(i))
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define fi first
#define se second
#define dbg(x) cout << #x " = " << ((x)) << endl
template <class T, class U>
ostream &operator<<(ostream &o, const pair<T, U> &p) {
  o << "(" << p.fi << "," << p.se << ")";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const vector<T> &v) {
  o << "[";
  for (T t : v) {
    o << t << ",";
  }
  o << "]";
  return o;
}

vector<string> split(const string &str, char sep) {
  vector<string> v;
  stringstream ss(str + sep);
  string buffer;
  while (getline(ss, buffer, sep))
    v.pb(buffer);
  return v;
}

int Na, Nt, Ng, Nc;
int m;

const int N = 52;
const ll mod = 1e9 + 7;

vector<string> x;
int X;

ll dp[N][N][N][N];
ll dfs(int a, int t, int g, int c) {
  if (a > Na || t > Nt || g > Ng || c > Nc)
    return 0;

  int idx = a + t + g + c;
  if (idx == X) {
    return (a == Na && t == Nt && g == Ng && c == Nc);
  }

  if (dp[a][t][g][c] >= 0)
    return dp[a][t][g][c];

  string f = x[idx];
  ll ret = 0;
  rep(i, f.size()) {
    if (f[i] == 'A')
      ret += dfs(a + 1, t, g, c);
    else if (f[i] == 'T')
      ret += dfs(a, t + 1, g, c);
    else if (f[i] == 'G')
      ret += dfs(a, t, g + 1, c);
    else if (f[i] == 'C')
      ret += dfs(a, t, g, c + 1);
    ret %= mod;
  }
  return dp[a][t][g][c] = ret;
}

int main() {
  cin >> Na >> Nt >> Ng >> Nc >> m;
  cin.ignore();

  map<string, int> t2id;
  vector<vector<string>> v(m);
  rep(i, m) {
    string s;
    getline(cin, s);
    vector<string> vs = split(s, ' ');

    string t = vs[0];
    t = t.substr(0, t.size() - 1);
    t2id[t] = i;

    for (int j = 1; j < vs.size(); ++j)
      v[i].pb(vs[j]);
  }

  // precalc for length
  vector<ll> L(m);
  for (int i = m - 1; i >= 0; --i) {
    rep(j, v[i].size()) {
      if (v[i][j][0] == '[')
        ++L[i];
      else {
        int idx = t2id[v[i][j]];
        L[i] += L[idx];
      }
    }
  }

  // Over Length
  if (L[0] > Na + Nt + Ng + Nc) {
    cout << 0 << endl;
    return 0;
  }

  // calc DP
  for (int i = m - 1; i >= 0; --i) {
    vector<string> new_v;
    rep(j, v[i].size()) {
      if (v[i][j][0] == '[')
        new_v.pb(v[i][j].substr());
      else {
        int idx = t2id[v[i][j]];
        for (string a : v[idx])
          new_v.pb(a);
      }
    }
    v[i] = new_v;
  }

  x = v[0];
  X = x.size();
  memset(dp, -1, sizeof(dp));
  cout << dfs(0, 0, 0, 0) << endl;
  return 0;
}


Error: MLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int(i) = 0; (i) < (int)(n); ++(i))
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define fi first
#define se second
#define dbg(x) cout << #x " = " << ((x)) << endl
template <class T, class U>
ostream &operator<<(ostream &o, const pair<T, U> &p) {
  o << "(" << p.fi << "," << p.se << ")";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const vector<T> &v) {
  o << "[";
  for (T t : v) {
    o << t << ",";
  }
  o << "]";
  return o;
}

vector<string> split(const string &str, char sep) {
  vector<string> v;
  stringstream ss(str + sep);
  string buffer;
  while (getline(ss, buffer, sep))
    v.pb(buffer);
  return v;
}

int Na, Nt, Ng, Nc;
int m;

const int N = 52;
const ll mod = 1e9 + 7;

vector<string> x;
int X;

ll dp[N][N][N][N];
ll dfs(int a, int t, int g, int c) {
  if (a > Na || t > Nt || g > Ng || c > Nc)
    return 0;

  int idx = a + t + g + c;
  if (idx == X) {
    return (a == Na && t == Nt && g == Ng && c == Nc);
  }

  if (dp[a][t][g][c] >= 0)
    return dp[a][t][g][c];

  string f = x[idx];
  ll ret = 0;
  rep(i, f.size()) {
    if (f[i] == 'A')
      ret += dfs(a + 1, t, g, c);
    else if (f[i] == 'T')
      ret += dfs(a, t + 1, g, c);
    else if (f[i] == 'G')
      ret += dfs(a, t, g + 1, c);
    else if (f[i] == 'C')
      ret += dfs(a, t, g, c + 1);
    ret %= mod;
  }
  return dp[a][t][g][c] = ret;
}

int main() {
  cin >> Na >> Nt >> Ng >> Nc >> m;
  cin.ignore();

  map<string, int> t2id;
  vector<vector<string>> v(m);
  rep(i, m) {
    string s;
    getline(cin, s);
    vector<string> vs = split(s, ' ');

    string t = vs[0];
    t = t.substr(0, t.size() - 1);
    t2id[t] = i;

    for (int j = 1; j < vs.size(); ++j)
      v[i].pb(vs[j]);
  }

  for (int i = m - 1; i >= 0; --i) {
    vector<string> new_v;
    rep(j, v[i].size()) {
      if (v[i][j][0] == '[')
        new_v.pb(v[i][j].substr());
      else {
        int idx = t2id[v[i][j]];
        for (string a : v[idx])
          new_v.pb(a);
      }
    }
    v[i] = new_v;
  }

  x = v[0];
  X = x.size();
  memset(dp, -1, sizeof(dp));
  cout << dfs(0, 0, 0, 0) << endl;
  return 0;
}