题解 | 小红的二叉树

//不能用cmath中的pow，会超过double的最大范围
#include <iostream>
#include <cmath>
using namespace std;
const int mod = 1e9+7;


int main() {
    int N;
    cin>>N;
    if(N<=2){
        switch (N) {
            case 1:cout<<0;return 0;
            case 2:cout<<1;return 0;
        }
    }

    long long ans = 1,yezi = 2;
    for(int i = 2;i<N;i++){

        ans = (ans+3*yezi)%mod;
        yezi = (yezi*2)%mod;
        //cout<<ans%mod<<endl;
    }
    cout<<ans%mod;
    return 0;
}