51NOD 1013(3的幂的和)

题目链接:传送门

题目大意:求(3^0+3^1+3^2+3^3+...+3^n)%1e9的值

题目思路:乘法逆元裸题

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <stack>
#include <cctype>
#include <queue>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <climits>
#define lson root<<1,l,mid
#define rson root<<1|1,mid+1,r
#define fi first
#define se second
#define ping(x,y) ((x-y)*(x-y))
#define mst(x,y) memset(x,y,sizeof(x))
#define mcp(x,y) memcpy(x,y,sizeof(y))
using namespace std;
#define gamma 0.5772156649015328606065120
#define MOD 1000000007
#define inf 0x3f3f3f3f
#define N 1000010
#define maxn 400005
typedef pair<int,int> PII;

long long n,m;
long long ksm(long long x,long long y){
    long long res=1;
    while(y){
        if(y&1)res=res*x%MOD;
        y>>=1;
        x=x*x%MOD;
    }
    return res;
}
int main(){
    int i,j,group,Case=0;
    while(cin>>n){
        long long temp=ksm(2,MOD-2);
        n=(ksm(3,n+1)-1+MOD)%MOD;
        cout<<(n*temp)%MOD<<endl;
    }
    return 0;
}