题目链接：满足条件的01序列

#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;a = (LL) a * a % p;k >>= 1;}return res;
}int main()
{int n;cin >> n;int a = 2 * n, b = n;int res = 1;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;cout << res << endl;return 0;
}