func_code_string: #include <bits/stdc++.h>   using namespace std;   template <class T>   using vec = vector<T>;   template <class T>   using vvec = vector<vec<T>>;   struct fast_ios {     fast_ios() {       cin.tie(0);       ios::sync_with_stdio(false);       cout << fixed << setprecision(20);     };   } fast_ios_;   template <class T>   inline bool chmax(T &a, T b) {     if (a < b) {       a = b;       return true;     }     return false;   }   template <class T>   inline bool chmin(T &a, T b) {     if (a > b) {       a = b;       return true;     }     return false;   }   template <typename T>   istream &operator>>(istream &is, vector<T> &vec) {     for (T &x : vec) is >> x;     return is;   }   template <typename T>   ostream &operator<<(ostream &os, const vector<T> &v) {     os <<  [ ;     for (auto _ : v) os << _ <<  ,  ;     os <<  ] ;     return os;   };   template <typename T>   ostream &operator<<(ostream &os, set<T> &st) {     os <<  ( ;     for (auto _ : st) {       os << _ <<  ,  ;     }     os <<  ) ;     return os;   }   template <typename T, typename U>   ostream &operator<<(ostream &os, const pair<T, U> &p) {     os <<  {  << p.first <<  ,   << p.second <<  } ;     return os;   }   template <typename T, typename U>   ostream &operator<<(ostream &os, const map<T, U> &mp) {     os <<  [ ;     for (auto _ : mp) {       os << _ <<  ,  ;     }     os <<  ]  << endl;     return os;   }   void dump_func() { cerr << endl; }   template <class Head, class... Tail>   void dump_func(Head &&head, Tail &&...tail) {     cerr << head;     if (sizeof...(Tail) > 0) {       cerr <<  ,  ;     }     dump_func(std::move(tail)...);   }   const int INF = (long long)1e9;   const long long INFLL = (long long)1e18 + 1;   const long long MOD = 1000000007;   const long double PI = acos(-1.0);   struct mint {     long long x;     mint(long long _x = 0) : x((_x % MOD + MOD) % MOD) {}     mint operator-() const { return mint(-x); }     mint &operator+=(const mint a) {       if ((x += a.x) >= MOD) x -= MOD;       return *this;     }     mint &operator-=(const mint a) {       if ((x += MOD - a.x) >= MOD) x -= MOD;       return *this;     }     mint &operator*=(const mint a) {       (x *= a.x) %= MOD;       return *this;     }     mint operator+(const mint a) const {       mint res(*this);       return res += a;     }     mint operator-(const mint a) const {       mint res(*this);       return res -= a;     }     mint operator*(const mint a) const {       mint res(*this);       return res *= a;     }     mint modpow(long long t) const {       if (!t) return 1;       mint a = modpow(t >> 1);       a *= a;       if (t & 1) a *= *this;       return a;     }     mint inv() const { return modpow(MOD - 2); }     mint &operator/=(const mint a) { return (*this) *= a.inv(); }     mint operator/(const mint a) const {       mint res(*this);       return res /= a;     }     friend std::ostream &operator<<(std::ostream &os, const mint &a) {       os << a.x;       return os;     }   };   int main() {     int N;     cin >> N;     string S;     cin >> S;     vector<mint> dp(4, 0);     dp[0] = 1;     for (int i = 0; i < int(N); ++i) {       vector<mint> nxt(4, 0);       if (S[i] ==  ? ) {         for (int _ = 0; _ < int(3); ++_)           for (int j = 0; j < int(4); ++j) nxt[j] += dp[j];         nxt[1] += dp[0];         nxt[2] += dp[1];         nxt[3] += dp[2];       } else {         for (int j = 0; j < int(4); ++j) nxt[j] += dp[j];         int from = 0;         if (S[i] ==  b ) from += 1;         if (S[i] ==  c ) from += 2;         nxt[from + 1] += dp[from];       }       swap(dp, nxt);     }     cout << dp[3] << endl;   }   