hdu2045

借鉴：：http://hi.baidu.com/sunnyykn/item/a71f3016269089ed9813d6ea

#include<iostream>using namespace std;int main(){    __int64 a[55] = {0};    a[1] = 3; a[2] = a[3] = 6;    for(int i = 4; i <= 50; i++)    {        a[i] = a[i-1] + 2*a[i-2];    }    int n;    while(cin >> n)        cout << a[n] << endl;}