给方格涂色,方格染色问题

排成一排n个方格，用三种不同颜色填充，求所有满足的涂法
采用递归
f(n) = f(n-1) + 2 *f(n-2)

#include<stdio.h>#include<stdlib.h>int main(void){int n,i;long long a[51]a[1] = 3;a[2] = 6;a[3] = 6;for(i=4;i<51;i++){a[i] = 2*a[i-2]+a[i-1];}while(scanf("%d",&n) != EOF){printf("%lldn",a[n]);}return 0;}