递推,注意为相邻柱子,文件尾
#include<iostream>
using namespace std;
int main(){
long long n,a[30]={0,2};
for(int i=2;i<=26;i++){
a[i]=3*a[i-1]+2;
}
while(cin>>n)
cout<<a[n]<<endl;
return 0;
}</iostream>