Yukicoder No.523 LED
solution
普通に組み合わせ。 $\sum_{0 \le i \lt n} {}_{2(n-i)}C_2 = \frac{2n!}{2^n}$。 $O(N)$。
implementation
#include <cstdio>
#include <vector>
#include <cassert>
#define repeat_from(i,m,n) for (int i = (m); (i) < int(n); ++(i))
using ll = long long;
using namespace std;
ll powmod(ll x, ll y, ll p) { // O(log y)
assert (0 <= x and x < p);
assert (0 <= y);
ll z = 1;
for (ll i = 1; i <= y; i <<= 1) {
if (y & i) z = z * x % p;
x = x * x % p;
}
return z;
}
ll inv(ll x, ll p) { // p must be a prime, O(log p)
assert ((x % p + p) % p != 0);
return powmod(x, p-2, p);
}
template <int mod>
int fact(int n) {
static vector<int> memo(1,1);
if (memo.size() <= n) {
int l = memo.size();
memo.resize(n+1);
repeat_from (i,l,n+1) memo[i] = memo[i-1] *(ll) i % mod;
}
return memo[n];
}
constexpr int mod = 1e9+7;
int main() {
int n; scanf("%d", &n);
int ans = fact<mod>(2*n) * inv(powmod(2, n, mod), mod) % mod;
printf("%d\n", ans);
return 0;
}