Submission #4027793

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Source Code Expand

#include <bits/stdc++.h>
typedef long long ll;

using namespace std;

template <typename T>
T mod_pow(T a, T b, T p) {
    if (b == 0) { return 1; }

    if (b % 2 == 0) {
		T d = mod_pow(a, b / 2, p);
		return (d * d) % p;
    } else {
		return (a * mod_pow(a, b - 1, p)) % p;
    }
    return 0;
}

template <typename T>
void ModConb(T& v, T& vi, ll mod) {
	v[0] = 1;
	vi[0] = 1;
	for (int i = 1; i < v.size(); i++) {
		v[i] = v[i - 1] * i;
		v[i] %= mod;

		vi[i] = mod_pow(v[i], mod - 2, mod);
		vi[i] %= mod;
	}
}


int main() {
	const ll MOD = 1e9 + 7;
	ll N, K;
	cin >> N >> K;

	vector<ll> v(N + K + 1);
	vector<ll> vi(N + K + 1);
	ModConb(v, vi, MOD);

	ll res = (((v[N - 1 + K] * vi[K]) % MOD) * vi[N - 1]) % MOD;
	cout << res << endl;

	return 0;
}

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