Submission #5437352

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int mo = 1e9 + 7, MAXN = 1e5 + 10;
template <class T>
inline void _read(T &x)
{
	x = 0;
	char t = getchar();
	while (!isdigit(t) && t != '-') t = getchar();
	if (t == '-')
	{
		_read(x);
		x *= -1;
		return ;
	}
	while (isdigit(t))
	{
		x = x * 10 + t - '0';
		t = getchar();
	}
}
ll n, k, f[MAXN];
inline ll fpm(ll a, ll b = mo - 2)
{
	if (b < 0) return 1;
	ll res = 1;
	while (b)
	{
		if (b & 1) res = res * a % mo;
		a = a * a % mo;
		b >>= 1;
	}
	return res;
}

int main()
{
	_read(n), _read(k);
	f[0] = 1ll;
	for(int i = 1; i < n; ++i)
	{
		for(int j = i; j; --j)f[j] = (f[j + 1] + f[j - 1]) % mo;
		f[0] = f[1];
	}
	printf("%lld", f[n - k] * fpm(2, n - k - 1) % mo);
	return 0;
}

Submission Info

Judge Result