HDOJ2045(环形染色问题)

#include <iostream> 
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <stack>
#include <math.h>


using namespace std;


__int64 a[55];


void cacu()
{
a[1] = 3;
a[2] = 6;
a[3] = 6;
for (int i = 4; i <= 50; i++) a[i] = a[i - 1] + a[i - 2] * 2;
}


int main()
{
int n;
cacu();
while (cin >> n)
{
cout << a[n] << endl;
}
}