hdu acm 2569

此题不懂，为什么是2*d[i-1] + d[i-2]

#include <cstdlib>
#include <iostream>
using namespace std;
int result[40];
int main()
{
  int C, n;
  result[1] = 3;
  result[2] = 9;
  for (int i=3; i<40; ++i)
	  result[i] = 2*result[i-1] + result[i-2];
  cin >> C;
  while (C--)
  {
    cin >> n;
    cout << result[n]<< endl;
  }  
}