結果

問題 No.823 Many Shifts Easy
コンテスト
ユーザー minami
提出日時 2019-04-26 22:17:29
言語 C++14
(gcc 13.3.0 + boost 1.87.0)
結果
AC  
実行時間 6 ms / 2,000 ms
コード長 3,953 bytes
コンパイル時間 1,535 ms
コンパイル使用メモリ 172,988 KB
実行使用メモリ 6,820 KB
最終ジャッジ日時 2024-11-25 04:36:43
合計ジャッジ時間 2,063 ms
ジャッジサーバーID
（参考情報） 		judge5 / judge4
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ファイルパターン 結果
other AC * 10
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ソースコード

diff # 

#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

//#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1'000'000'007;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

template<typename T>
vector<T> gen_v(size_t a) { return vector<T>(a); }
template<typename T, typename ...Ts>
auto gen_v(size_t a, Ts... ts) { return vector<decltype(gen_v<T>(ts...))>(a, gen_v<T>(ts...)); }
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T &t, const V &v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T &t, const V &v) { for (auto &e : t) fill_v(e, v); }

template<int MOD>
struct ModInt {
	static const int kMod = MOD;
	unsigned x;
	ModInt() :x(0) {}
	ModInt(signed x_) { x_ %= MOD; if (x_ < 0)x_ += MOD; x = x_; }
	ModInt(signed long long x_) { x_ %= MOD; if (x_ < 0)x_ += MOD; x = x_; }
	int get()const { return (int)x; }
	ModInt &operator+=(ModInt m) { if ((x += m.x) >= MOD)x -= MOD; return *this; }
	ModInt &operator-=(ModInt m) { if ((x += MOD - m.x) >= MOD)x -= MOD; return *this; }
	ModInt &operator*=(ModInt m) { x = (unsigned long long)x*m.x%MOD; return *this; }
	ModInt &operator/=(ModInt m) { return *this *= m.inverse(); }
	ModInt operator+(ModInt m)const { return ModInt(*this) += m; }
	ModInt operator-(ModInt m)const { return ModInt(*this) -= m; }
	ModInt operator*(ModInt m)const { return ModInt(*this) *= m; }
	ModInt operator/(ModInt m)const { return ModInt(*this) /= m; }
	ModInt operator-()const { return ModInt(MOD - (signed)x); }
	bool operator==(ModInt m)const { return x == m.x; }
	bool operator!=(ModInt m)const { return x != m.x; }
	ModInt inverse()const {
		signed a = x, b = MOD, u = 1, v = 0;
		while (b) {
			signed t = a / b;
			a -= t * b; swap(a, b);
			u -= t * v; swap(u, v);
		}
		if (u < 0)u += MOD;
		return ModInt(u);
	}
};
template<int MOD>
ostream &operator<<(ostream &os, const ModInt<MOD> &m) { return os << m.x; }
template<int MOD>
istream &operator>>(istream &is, ModInt<MOD> &m) { signed long long s; is >> s; m = ModInt<MOD>(s); return is; };

template<int MOD>
ModInt<MOD> pow(ModInt<MOD> a, unsigned long long k) {
	ModInt<MOD> r = 1;
	while (k) {
		if (k & 1)r *= a;
		a *= a;
		k >>= 1;
	}
	return r;
}

using mint = ModInt<MOD>;

vector<mint> fact, factinv, inv;
void precompute(int n) {
	int m = fact.size();
	if (n < m)return;
	n = min(n, mint::kMod - 1); //  N >= kMod  =>  N! = 0 (mod kMod)
	fact.resize(n + 1);
	factinv.resize(n + 1);
	inv.resize(n + 1);
	if (m == 0) {
		fact[0] = 1;
		m = 1;
	}
	for (int i = m; i <= n; i++)
		fact[i] = fact[i - 1] * i;
	factinv[n] = fact[n].inverse();
	for (int i = n; i >= m; i--)
		factinv[i - 1] = factinv[i] * i; // ((i-1)!)^(-1) = (i!)^(-1) * i
	for (int i = m; i <= n; i++)
		inv[i] = factinv[i] * fact[i - 1];
}

mint C(int n, int k) {
	if (k < 0 || n < k)return 0;
	// Lucas's theorem
	if (n >= mint::kMod)
		return C(n % mint::kMod, k % mint::kMod) * C(n / mint::kMod, k / mint::kMod);
	precompute(n);
	return k > n ? 0 : fact[n] * factinv[n - k] * factinv[k];
}

mint P(int n, int k) {
	if (k < 0 || n < k)return 0;
	precompute(n);
	return k > n ? 0 : fact[n] * factinv[n - k];
}

mint H(int n, int k) {
	if (n == 0 && k == 0)return 1; // H(0,0) = 1 != C(-1,0) = 0
	return C(n + k - 1, k);
}

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, K; cin >> N >> K;
	mint ans = 0;
	precompute(N);
	rep(i, 1, N) {
		ans += mint(i) * (C(K, 2) * P(N - 2, K - 2) + P(N - 1, K));
	}
	ans += mint(N) * P(N - 1, K);
	cout << ans << endl;

	return 0;
}
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