Submission #7115801

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Source Code Expand

#include <algorithm>
#include <iostream>
#include <vector>
#include <map>
using namespace std;
const int MOD = (int)1e9 + 7;

//---------------------------------------------------------------
template<int MOD> struct ModInt { int x;
    explicit operator bool() const { return !!x; }
    ModInt(int v = 0) : x(v % MOD) { if (x < 0) x += MOD; }
    ModInt(long long v) : x(v % MOD) { if (x < 0) x += MOD; }
    ModInt &operator+=(const ModInt &r) { if ((x += r.x) >= MOD) x -= MOD; return *this; }
    ModInt &operator-=(const ModInt &r) { if ((x += MOD - r.x) >= MOD) x -= MOD; return *this; }
    ModInt &operator*=(const ModInt &r) { x = 1LL * x * r.x % MOD; return *this; }
    ModInt &operator/=(const ModInt &r) { return *this *= r.inv(); }
    ModInt operator-() const { return x ? ModInt(MOD - x) : ModInt(x); }
    ModInt operator+(const ModInt &r) const { return ModInt(*this) += r; }
    ModInt operator-(const ModInt &r) const { return ModInt(*this) -= r; }
    ModInt operator*(const ModInt &r) const { return ModInt(*this) *= r; }
    ModInt operator/(const ModInt &r) const { return ModInt(*this) /= r; }
    ModInt inv() const { long long a = x, b = MOD, u = 1, v = 0;
        while (b) { long long t = a / b; a -= t * b; swap(a, b); u -= t * v; swap(u, v); }
        return ModInt(u); } // (*this).pow(MOD - 2);
    ModInt pow(long long k) const { ModInt r(1), a(x);
        while (k) { if (k & 1) r *= a; a *= a; k >>= 1; } return r; }
    bool operator==(const ModInt r) const { return x == r.x; }
    bool operator!=(const ModInt r) const { return x != r.x; }
    bool operator< (const ModInt r) const { return x <  r.x; }
    friend ostream& operator<<(ostream &os, const ModInt<MOD>& a) { return os << a.x; }
    friend istream& operator>>(istream &is, ModInt<MOD>& a) { return is >> a.x; }
};
template<typename T, int SZ> struct Comb { vector<T> _fac, _ifac, _inv;
    Comb() : _fac(SZ + 1), _ifac(SZ + 1), _inv(SZ + 1) {
        _fac[0] = _ifac[SZ] = _inv[0] = 1;
        for (int i = 1; i <= SZ; i++) _fac[i] = _fac[i - 1] * i;
        _ifac[SZ] /= _fac[SZ];
        for (int i = SZ - 1; i >= 0; i--) _ifac[i] = _ifac[i + 1] * (i + 1);
        for (int i = 1; i <= SZ; i++) _inv[i] = _ifac[i] * _fac[i - 1]; }
    T inv(int n) { return n < 0 ? T(0) : _inv[n]; }
    T fac(int n) { return n < 0 ? T(0) : _fac[n]; }
    T ifac(int n) { return n < 0 ? T(0) : _ifac[n]; }
    T P(int a, int b) { return (b < 0 || a < b) ? T(0) : _fac[a] * _ifac[a - b]; }
    T C(int a, int b) { return b < 0 ? T(0) : P(a, b) * _ifac[b]; }
    T H(int n, int k) { if (n < 0 || k < 0) return T(0);
        return k == 0 ? T(1) : C(n + k - 1, k); }
    T S(int n, int k) { T r = 0;
        for (int i = 0; i <= k; i++) {
            T t = C(k, i) * T(i).pow(n); r += ((k - i) & 1 ? -t : t);
        }
        return r * _ifac[k];
    }
    T B(int n, int k) {
        if (n == 0) return T(1);
        T r = 0; k = min(k, n);
        vector<T> dp(k + 1); dp[0] = T(1);
        for (int i = 1; i <= k; i++) dp[i] = dp[i - 1] + (i & 1 ? -_ifac[i] : _ifac[i]);
        for (int i = 1; i <= k; i++) r += T(i).pow(n) * _ifac[i] * dp[k - i];
        return r;
    }
};
typedef ModInt<MOD> mint;
//---------------------------------------------------------------

template<class T> ostream& operator<<(ostream& os, const vector<T>& vec) { for (auto &vi: vec) os << vi << " "; return os; }

int main() {
    Comb<mint, 200000> com;
    int n; cin >> n;
    string s; cin >> s;
    map<pair<int, int>, mint> dp;
    dp[{0, 0}] = 1;
    for (int i = 0; i < n * 2; i++) {
        map<pair<int, int>, mint> dp2;
        for (auto &p: dp) {
            auto &st = p.first;
            int op = st.first, cl = st.second;
            mint cur = p.second;
            if (s[i] == 'W') {
                if (op % 2 == 0) {  // close
                    if (cl + 1 <= n && op) dp2[{op - 1, cl + 1}] += cur * op;
                } else {            // open
                    if (op + 1 <= n) dp2[{op + 1, cl}] += cur;
                }
            }
            if (s[i] == 'B') {
                if (op % 2 == 1) {  // close
                    if (cl + 1 <= n && op) dp2[{op - 1, cl + 1}] += cur * op;
                } else {            // open
                    if (op + 1 <= n) dp2[{op + 1, cl}] += cur;
                }
            }
        }
        swap(dp, dp2);
    }
    cout << dp[{0, n}] * com.fac(n) << endl;
    return 0;
}

Submission Info

Judge Result