卡特兰数

卡特兰数：https://oi-wiki.org/math/catalan/

#include <iostream>
using namespace std;

const int mod = 1e9+7;

long long qmi(long long a, long long k, long long p)
{
    long long res = 1;
    while (k)
    {
        if (k & 1) res = res * a % p;
        a = a * a % p;
        k >>= 1;
    }
    return res;
}

int main(){
    long long n;cin >> n;
    long long res = 1;
    for(int i = 2*n;i > n;--i) res = res * i % mod;
    for(int i = 1;i <= n;++i) res = res * qmi(i, mod-2, mod) % mod;
    res = res * qmi(n+1, mod-2, mod) % mod;
    cout << res << endl;
    return 0;
}