题解：http://vfleaking.blog.163.com/blog/static/17480763420119685112649/
#include <iostream>
#include <cstdio>
using namespace std;
struct data{int a[101],len;};
data mul(data a,int k){
for(int i=1;i<=a.len;i++)a.a[i]*=k;
for(int i=1;i<=a.len;i++)a.a[i+1]+=a.a[i]/10,a.a[i]%=10;
if(a.a[a.len+1]!=0)a.len++;
return a;
}
data sub(data a,data b){
a.a[1]+=2; int j=1;
while(a.a[j]>=10){a.a[j]%=10;a.a[j+1]++;j++;}
for(int i=1;i<=a.len;i++){
a.a[i]-=b.a[i];
if(a.a[i]<0){a.a[i]+=10;a.a[i+1]--;}
}
while(a.a[a.len]==0)a.len--;
return a;
}
int main(){
data f[101]; f[1].a[1]=1;f[2].a[1]=5;
f[1].len=f[2].len=1;
int n; scanf("%d",&n);
for(int i=3;i<=n;i++)f[i]=sub(mul(f[i-1],3),f[i-2]);
for(int i=f[n].len;i>0;i--)printf("%d",f[n].a[i]);
return 0;
}