AcWing 889. 满足条件的01序列
原题链接
简单
作者:
那就揣兜兜噢
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2022-01-26 17:28:15
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所有人可见
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阅读 145
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
const int mod = 1e9 + 7, N = 100010;
int qmi(int a, int b, int p)
{
int res = 1;
while(b)
{
if(b & 1) res = (LL)res * a % p;
a = (LL) a * a % p;
b >>= 1;
}
return res;
}
int main()
{
int n;
cin >> n;
int a = 2 * n, b = n;
int res = 1;
for(int i = 1, j = a; i <= b; i ++, j --)
{
res = (LL)res * j % mod;
res = (LL)res * qmi(i,mod - 2,mod) % mod;
}
res = (LL)res * qmi(n + 1,mod - 2,mod) % mod ;
cout << res << endl;
return 0;
}