AcWing 889. 满足条件的01序列
原题链接
简单
作者:
我要出去乱说
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2021-01-22 11:19:47
,
所有人可见
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阅读 195
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
const int mod = 1e9 + 7;
int qmi(int a, int k, int p)
{
int res = 1;
while (k)
{
if (k & 1) res = (LL)res * a % p;
k >>= 1;
a = (LL)a * a % p;
}
return res;
}
int main()
{
int n;
cin >> n;
int a = 2 * n, b = n;
int res = 1;
//计算组合数C(n, 2n)
for (int i = a; i > a - b; i -- ) res = (LL)res * i % mod;
for (int i = 1; i <= b; i ++ ) res = (LL)res * qmi(i, mod - 2, mod) % mod;
res = (LL)res * qmi(n + 1, mod - 2, mod) % mod; //计算卡特兰数C(n, 2n) / (n +1)
cout << res << endl;
return 0;
}