牛客网小a的旅行计划

地址:https://ac.nowcoder.com/acm/contest/223/B

此题是一道套公式题目，不难，ans=(pow(4,n+1)-4*pow(3,n+1)+6*pow(2,n+1)-4)*pow(8,mod-2)

#include <cstdio>
#include <queue>
#include <cstring>
#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <map>
#include <cmath>
using namespace std;
typedef  long long ll;
const ll mod=1e8+7;
ll  qow(ll a,ll b)
{
    if(b==0)return 1;
    if(b==1)return a%mod;
    ll sum=qow(a,b/2);
    return b%2?(a*((sum*sum)%mod))%mod:(sum*sum)%mod;
}
int main()
{
    ll n;
    cin>>n;
    n++;
    ll ans=((((qow(4,n)-4*qow(3,n)+6*qow(2,n)-4)%mod+mod)%mod)*qow(8,mod-2))%mod;
    cout<<ans<<endl;
    return 0;
}