#include <iostream> using namespace std; long long a[51]; int main() { int n; a[1] = 3; a[2] = 6; a[3] = 6; for (int i = 4; i <= 50; i++) { a[i] = a[i - 1] + 2 * a[i - 2]; } while (cin >> n) { cout << a[n] << endl; } }
原文：http://www.cnblogs.com/theskulls/p/6011575.html