【51Nod】1013 - 3的幂的和（等比数列 & 逆元）

输入一个数N(0 <= N <= 10^9)

输出：计算结果

3

40

#include <stdio.h>
#include <cstring>
#include <algorithm>
using namespace std;
#define CLR(a,b) memset(a,b,sizeof(a))
#define INF 0x3f3f3f3f
#define LL long long
const LL mod = 1000000007;
LL quick_mod(LL a,LL n)
{
	LL ans = 1;
	while (n)
	{
		if (n & 1)
			ans = ans * a % mod;
		a = a * a % mod;
		n >>= 1;
	}
	return ans;
}
int main()
{
	LL n;
	scanf ("%lld",&n);
	LL ans = quick_mod(3,n+1);
	ans--;
	LL inv = quick_mod(2,mod-2);
	ans = ans * inv % mod;
	printf ("%lld\n",ans);
	return 0;
}