Language: C++

#include <bits/stdc++.h>
#define LOOP(n) for (int _i = 0; _i < (n); _i++)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, r, n) for (int i = (r); i < (n); ++i)
#define ALL(obj) begin(obj), end(obj)
using namespace std;
using ll = long long;
using ull = unsigned long long;

const int INF = 2100100100;
const int MOD = 1e9 + 7;

// 多次元 vector 生成
template <class T>
vector<T> make_vec(size_t a) {
    return vector<T>(a);
}
template <class T, class... Ts>
auto make_vec(size_t a, Ts... ts) {
    return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}
template <class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return 1;
    }
    return 0;
}

template <int mod>
struct ModInt {
    int val;
    ModInt() : val(0) {}
    ModInt(long long x) : val(x >= 0 ? x % mod : (mod - (-x) % mod) % mod) {}
    int getmod() { return mod; }
    ModInt &operator+=(const ModInt &p) {
        if ((val += p.val) >= mod) {
            val -= mod;
        }
        return *this;
    }
    ModInt &operator-=(const ModInt &p) {
        if ((val += mod - p.val) >= mod) {
            val -= mod;
        }
        return *this;
    }
    ModInt &operator*=(const ModInt &p) {
        val = (int)(1LL * val * p.val % mod);
        return *this;
    }
    ModInt &operator/=(const ModInt &p) {
        *this *= p.inverse();
        return *this;
    }
    ModInt operator-() const { return ModInt(-val); }
    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
    bool operator==(const ModInt &p) const { return val == p.val; }
    bool operator!=(const ModInt &p) const { return val != p.val; }
    ModInt inverse() const {
        int a = val, b = mod, u = 1, v = 0, t;
        while (b > 0) {
            t = a / b;
            swap(a -= t * b, b);
            swap(u -= t * v, v);
        }
        return ModInt(u);
    }
    ModInt pow(long long n) const {
        ModInt ret(1), mul(val);
        while (n > 0) {
            if (n & 1) ret *= mul;
            mul *= mul;
            n >>= 1;
        }
        return ret;
    }
    friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.val; }
    friend istream &operator>>(istream &is, ModInt &a) {
        long long t;
        is >> t;
        a = ModInt<mod>(t);
        return (is);
    }
    static int get_mod() { return mod; }
};
using modint = ModInt<MOD>;

string S;

int main() {
    // cin.tie(0);
    // ios::sync_with_stdio(false);

    cin >> S;
    int n = S.size();
    vector<int> A(n + 1), B(n + 1), C(n + 2), L(n + 1), R(n + 2);

    FOR(i, 0, n) {
        A[i + 1] = A[i];
        B[i + 1] = B[i];
        C[n - i] = C.[n + 1 - i];
        L[i + 1] = L[i];
        R[n - i] = R[n + 1 - i];
        if (S[i] == 'A') {
            A[i + 1] += 1;
        } else if (S[i] == 'B') {
            B[i + 1] += 1;
        } else if (S[i] == '?') {
            L[i + 1] += 1;
        }

        if (S[n - 1 - i] == '?') {
            R[n - i] += 1;
        } else if (S[n - 1 - i] == 'C') {
            C[n - i] += 1;
        }
    }

    /*
    REP(i, n + 1) cout << A[i] << " ";
    cout << endl;
    REP(i, n + 1) cout << B[i] << " ";
    cout << endl;
    REP(i, n + 1) cout << C[i] << " ";
    cout << endl;
    REP(i, n + 1) cout << L[i] << " ";
    cout << endl;
    REP(i, n + 1) cout << R[i] << " ";
    cout << endl;
    */

    modint ans = modint(0);
    FOR(i, 2, n) {
        if (S[i - 1] == 'B' || S[i - 1] == '?') {
            ans += modint(3).pow(L[i - 1] + R[i + 1]) * modint(A[i - 1]) * modint(C[i + 1]);
            ans += modint(3).pow(L[i - 1] + R[i + 1] - 1) * modint(L[i - 1]) * modint(C[i + 1]);
            if (L[i - 1] + R[i + 1] > 0)
                ans += modint(3).pow(L[i - 1] + R[i + 1] - 1) * modint(A[i - 1]) * modint(R[i + 1]);
            if (L[i - 1] + R[i + 1] > 1)
                ans += modint(3).pow(L[i - 1] + R[i + 1] - 2) * modint(L[i - 1]) * modint(R[i + 1]);
        }
    }
    cout << ans << endl;

    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:114:22: error: expected unqualified-id before '[' token
  114 |         C[n - i] = C.[n + 1 - i];
      |                      ^